All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have already issued Fatwa: 83361 clarifying that people in this group that you inquired about in your question are not Muslims because they deny what is proven from religion as being an absolute truth and deny some pillars of faith like the belief in the angels and the resurrection. Allaah says (which means): {Those who disbelieve have claimed that they will never be resurrected. Say, "Yes, by my Lord, you will surely be resurrected; then you will surely be informed of what you did. And that, for Allaah, is easy".}[Quran 64:7]. So this group denies the resurrection and other pillars of faith, and since it is the case, the prayer (when being led by them) is not valid because they are disbelievers.
Therefore, you have to stay away from this group because you might be influenced by their misconceptions and falsehood. The Prophet said: "The example of a good companion (who sits with you) in comparison with a bad one, is like that of the musk seller and the blacksmith's bellows (or furnace); from the first you would either buy musk or enjoy its good smell while the bellows would either burn your clothes or you get a bad nasty smell thereof." [Al-Bukhari and Muslim] The Prophet further said: "Do not befriend except a good believer and do not share your food except with a pious person." [At-Tirmithi] So you have to avoid keeping company with them and limit your company to pious and righteous men only.
Allaah knows best.