A word for a group of people in a church

The word I would use is congregation:

Full Definition of CONGREGATION 1 a : an assembly of persons : gathering; especially : an assembly of persons met for worship and religious instruction. - Merriam Webster

A more poetic answer would be "flock." Which is used metaphorically in Christianity when to Jesus is referred to as the Good Shepard. Otherwise congregation is the best, more descriptive and commonly used word.

This reference from the US Conference of Catholic Bishops seems to use ministers as a term that would include ordained celebrants and lay participants assisting in the mass. But they're pretty adamant the only celebrants are ordained.

In a religious gathering, conclave--whatever you want to call it-- where the participants are peers, though, I think you could call everyone celebrants, though perhaps that is because I am North American.

All that said, I think the other answer of congregation/congregants is more appropriate, especially if everyone attending is to be included.

I would use parish or parishioners. Otherwise, I think that Margana's congregation is another good choice.

According to Collins:

parish

noun

1) a subdivision of a diocese, having its own church and a clergyman related adjective parochial
2) the churchgoers of such a subdivision