Like a lot of these questions, It depends on what the league rules are. Most leagues define a legal lineup as at least 2 regular members.

Also there is a difference between subs and roster members.

Subs are not regular members, but roster members are.

If your league says regulars and sub count toward a legal lineup then it's legal.

as for them not having bowled before, unless theres a rule about it  then it's legal.

some leagues have a rule that say a new person can't start on positions rounds or last two week.

Our league rules just state 3 bowlers for a legal lineup. There is also a rule that prevents a bowler from bowling in a position round unless they have a certain number of games. 

That answers your own question. If it's not a position round and the rules just states three bowlers, then two reg. and a sub. is legal.

Yes 5 person team.

The reason I was asking is 2 weeks ago this team forfeited with only 1 bowler. This week they had 2 and talked someone into bowling with them as a sub for what I am assuming is just the 1 week. It's kind of like going to a baseball game and a team is 1 player short so they pick someone out of the stands to play.

Without a league rule stating otherwise, they have 3 legal bowlers so they have a legal lineup.

Something we need to discuss at next years league meeting.

3 makes it legal