I've never heard of prayer groups not accepting Catholics, at least not in our area.
Nor I. This was a young catholic housewife up by the Chicago area who was telling my sister and me (non-catholics - I later converted) that they didn't want her in their prayer group because she was catholic. I have no reason to believe she was not telling the truth.
It made me feel very sad to hear her tell that; we were on our way to bury my aunt in a distant town and plenty of time to gab in the car.
I do know there is still some bigotry on both sides (and other sides) which I can't deal with any more and don't want anything to do with any of it. The thoughts of many hearts are revealed on the internet.
My birth religion has fallen into disfavor (Methodism), but one very positive thing is that as I child I was never taught that anyone was going to hell because of their religion, and I was never taught to hate anyone for their religion or race, either at home or at church. Some of my little playmates of other denominations were not being taught the same I now know. Now I find out on the internet that if you don't believe just right, many people think you are going to hell. It's enough to turn you off of religion altogether, especially after what has happened with Islam.
"I've never heard of prayer groups not accepting Catholics, at least not in our area."
Yeah, as long as they keep their Catholic beliefs to themselves. Inevitably, that is what is going to happen. The Protestant group will try to convert the Catholic of the "error" of his ways. If the Catholic knows his faith, you will see how quickly the Catholic is asked to leave. I know this from experience.
Regards