Question:
How do you go about either increasing your youth group or starting a new one?

Answer:
Make it fun.

Have activities along with a Bible study.
If the Lord’s stirring your spirit to have a youth group or increase the one you have, talk to someone, maybe the Lord is stirring their heart too to help in this work.
Assume there is interest.
“NOW THEN DO IT” (2 Samuel 3:18)!