Start small, but very public. New ministries need lots of visibility. This is especially true with young adults since they're such a diverse group. Many are in school, living away from home, or living at home and working full-time. Simply keeping an accurate mailing list of this age group is an undertaking.
Publicity is key.
Try something which gathers together individuals who had a former connection with the church. Easy access, thankful exits. Make sure young adults feel they are welcome and that their involvement in worship and programs is intentional and appreciated. Also, be careful to welcome the short- term nature of their involvement. Avoid activities which require weekly attendance for them to feel like full attendants.