Enjoining what is good and forbidding what is evil is one of the greatest duties that Islam enjoins upon Muslims; Allaah has made it the main quality of this ummah and a condition of it being the best of people. Allaah says (interpretation of the meaning): 

“You (true believers in Islamic Monotheism, and real followers of Prophet Muhammad and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al‑Munkar (polytheism, disbelief and all that Islam has forbidden), and you believe in Allaah”

[Aal ‘Imraan 3:110] 

“The believers, men and women, are Awliyaa’ (helpers, supporters, friends, protectors) of one another; they enjoin (on the people) Al‑Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do), and forbid (people) from Al‑Munkar (i.e. polytheism and disbelief of all kinds, and all that Islam has forbidden); they perform As-Salaah (Iqaamat-as-Salaah), and give the Zakaah, and obey Allaah and His Messenger. Allaah will have His Mercy on them. Surely, Allaah is All-Mighty, All-Wise”

[al-Tawbah 9:71]

See also question no. 11403. This is indicative of the high status that Islam gives to enjoining what is good and forbidding what is evil. 

With regard to the problems of undertaking this duty with regard to those who are older than you, especially your father, you can overcome these problems to a great extent by understanding that this duty must be accompanied by three basic characteristics: knowledge, kindness and patience. Shaykh al-Islam Ibn Taymiyah said: “These three are essential: knowledge, kindness and patience: knowledge before enjoining or forbidding, kindness when doing so and patience afterwards, as it was narrated that one of the salaf said: No one can enjoin what is good or forbid what is evil unless he understands what he is enjoining or forbidding, and is kind in enjoining and kind in forbidding, and is patience and forbearing when enjoining and when forbidding.” 

Majmoo’ al-Fataawa, 28/137 

In order to solve these problems we should first draw attention to the importance of being patient and forbearing. Do not be too harsh with your father, because he is first of all your father, and because he is older than you. Seek help in doing that by choosing the right time for each conversation, watching out for times when he is in a good mood and is more likely to be influenced and respond. Also seek the help of wise, righteous and sincere people. Before all that it is essential to be sincere towards him in word and deed, and in all your affairs, and to fear that the punishment of his Lord may befall him, and after that be patient in awaiting the decree of your Lord. Whatever happens to you if you do this duty for the sake of Allaah, as Luqmaan the wise said to his son: 

“O my son! Aqim‑is‑Salaah (perform As‑Salaah), enjoin (on people) Al‑Ma‘roof  (Islamic Monotheism and all that is good), and forbid (people) from Al‑Munkar (i.e. disbelief in the Oneness of Allaah, polytheism of all kinds and all that is evil and bad), and bear with patience whatever befalls you. Verily, these are some of the important commandments (ordered by Allaah with no exemption)”

[Luqmaan 31:17 – interpretation of the meaning]

You should always remember the Close Friend of the Most Merciful, Ibraaheem, and how he addressed his father when he told him not to associate others with Allaah, which is worse than the things that your father is doing; how he was loving towards him, felt pity for him and was sincere towards him, then he bore his bad response with patience. Allaah says (interpretation of the meaning): 

“And mention in the Book (the Qur’aan) Ibraaheem (Abraham). Verily, he was a man of truth, a Prophet.

42. When he said to his father: ‘O my father! Why do you worship that which hears not, sees not and cannot avail you in anything?

43. ‘O my father! Verily, there has come to me of the knowledge that which came not unto you. So follow me, I will guide you to the Straight Path.

44. ‘O my father! Worship not Shaytaan (Satan). Verily, Shaytaan (Satan) has been a rebel against the Most Gracious (Allaah).

45. ‘O my father! Verily, I fear lest a torment from the Most Gracious (Allaah) should overtake you, so that you become a companion of Shaytaan (Satan) (in the Hell‑fire).’

46. He (the father) said: ‘Do you reject my gods, O Ibraaheem (Abraham)? If you stop not (this), I will indeed stone you. So get away from me safely (before I punish you).’

47. Ibraaheem (Abraham) said: ‘Peace be on you! I will ask forgiveness of my Lord for you. Verily, He is unto me Ever Most Gracious.

48. ‘And I shall turn away from you and from those whom you invoke besides Allaah. And I shall call upon my Lord and I hope that I shall not be unblest in my invocation to my Lord’”

[Maryam 19:41-48] 

With regard to the group that you are leading, this is an easier matter, because their closeness in age makes it easier for them to accept advice from you, especially if they know you to be sincere and honest, and that you practise what you preach. This practical implementation of what you are calling them to is one of the most effective means of da’wah, especially if speaking directly will cause them to turn away and be stubborn. 

With regard to your advice and da’wah being undermined by their parents’ directions, you should avoid blatantly contradicting what they say; rather you should look at the mistakes and teach them the right way. If it is the matter of a sin that they are committing, then teach them to fear disobeying Allaah and to love obeying Him. Instill in their hearts a sense of the greatness of Allaah and the seriousness of His prohibitions, then teach them about what has been narrated concerning these particular sins. This is what you should do if it is the matter of an innovation that you notice them engaging in. First instill in their hearts the love of the Sunnah of the Prophet (peace and blessings of Allaah be upon him) and teach them that this is the practical manifestation of love for the Prophet (peace and blessings of Allaah be upon him). And instill in their hearts a hatred for bid’ah and tell them how it is the way that leads to Hell even though the one who follows it thinks that it is the way that leads to Paradise. Tell them of the advice of the Prophet (peace and blessings of Allaah be upon him) to his ummah concerning that, as narrated in the hadeeth of al-‘Irbaad ibn Saariyah (may Allaah be pleased with him) who said: 

“The Messenger of Allaah (peace and blessings of Allaah be upon him) led us in praying Fajr then he turned to us and delivered an eloquent exhortation that caused our eyes to flow with tears and our hearts to melt. We said (or they said): ‘O Messenger of Allaah, it is as if this exhortation is a farewell, so advise us.’ He said: ‘I advise you to fear Allaah and to listen and obey even if (your leader is) an Abyssinian slave. Those of you who live will see great differences after I am gone, so I urge you to adhere to my Sunnah and the path of the Rightly-Guided Caliphs, and take hold of it firmly. And beware of newly-invented matters, for every newly-invented matter is an innovation and every innovation is a going astray.’” Narrated by Ahmad, 16692 and others; classed as saheeh by al-Albaani in al-Saheehah, 45. 

So after this general advice we move to addressing the specific bid’ah into which they have fallen. 

It should be noted that your presence in this group should be for a good purpose; as long as your presence is more likely to do good or to reduce any evils, then join them in their gatherings with this intention. But beware of letting your presence have any negative effect or make you get used to accepting mistakes, or lead you to make similar mistakes. In that case you should leave the place where evil things are happening. Allaah says (interpretation of the meaning): 

“And it has already been revealed to you in the Book (this Qur’aan) that when you hear the Verses of Allaah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them. Surely, Allaah will collect the hypocrites and disbelievers all together in Hell”

[al-Nisa’ 4:140]

Shaykh Ibn Sa’di (may Allaah have mercy on him) said: Whoever attends a gathering in which Allaah is disobeyed, then he must denounce their action if he is able to, or he must leave if he not able to do that.

Tafseer Ibn Sa’di, p. 210. 

May Allaah help us and you to do that which He loves and which pleases Him, and make us steadfast in adhering to true guidance and the straight path.