Praise be to Allah.
There is nothing wrong with your attending this party with the intention of calling people to Allaah and trying to influence these new Muslims, if that is possible, and so long as your attending will not have any bad consequences that outweigh the good you hope to achieve. The interests that may be served by your attending include the following:
1-Saving these sisters and other new Muslims from misguidance.
2-Establishing some friendly contacts between you and them which may help make it easier for you to advise them and guide them.
3-Proclaiming the truth and making the word of Allaah prevail.
4-Ensuring that you will be free from guilt before Allaah and that you have done that which Allaah has enjoined upon you. “And when a community among them said: ‘Why do you preach to a people whom Allaah is about to destroy or to punish with a severe torment?’ (The preachers) said: ‘In order to be free from guilt before your Lord (Allaah), and perhaps they may fear Allaah’” [al-A’raaf 7:164 – interpretation of the meaning]
The bad consequences that may result from your attending include:
1-The people organizing the party and the new Muslims may think that you agree with their views, which will be a source of fitnah or misguidance to them.
2-You may see some evil things and not be able to denounce them, such as if they openly pronounce their misguided views during the party and call people to them.
3-You may be influenced by some of what you see, so that you may agree with them or be inclined towards some of their opinions.
You may be able to avoid these bad consequences.
Whatever the case, you have to weigh up the pros and cons. If the interests served by your attending will outweigh any feared bad consequences, then seek the help of Allaah and attend the gathering and strive to proclaim the truth with wisdom and beautiful preaching.
If it seems to you after you attend that the bad consequences were greater, then you have to forget about these gatherings altogether.
We ask Allaah to guide us and help us to do and say that which is right. And Allaah knows best.