Learn English – Words to address young (male) people with

I think the best you can do here is

A young couple emerged from the shadows.

If you need to specify gender, I would use young man or young woman as appropriate. While I agree with Charles that in spoken English young man is used more towards children, I would posit that this is the case only when addressing them directly. I could, for example, address a 15 year old boy as young man. It would make me sound professorial, but it would not be out of place.

However, if someone were to tell me they

saw a young man entering the house

I would think of a young adult, not a child.

Update:

The closest to guy would be youth but it only applies to males when used to refer to individuals as opposed to groups. Better, but also exclusively male, is lad but that is also more British than American. Kid can be used, but is ambiguous. An older person can refer to young adults as kids but it usually means children. Finally, and probably best in your case, you can use youngster(s). Once more, though, this is a term that is used by older people towards younger ones. It is more used for teenagers and young adults than for actual children but it also carries an implication that the person using the term is older than those she is describing.

A congregant is a a member of a congregation, so I might say “a fellow congregant,” though that does sound quite formal.

Here is Merriam-Webster's entry:

congregant n | con·gre·gant | \ˈkäŋ-gri-gənt\
: a person who is part of a congregation : a person who is attending religious services or who regularly attends religious services
: one who congregates; specifically : a member of a congregation

It's similar to parishioner. See this question for discussion of the two.