Why not encourage those in your young adult ministry to help coordinate a special Mass for their community? Encouraging participation in the Mass could help bond the young adults and serve as a good reminder of the main reason why they’re there: to live out their faith with other young adults. Make a sign-up sheet for lectors and Eucharistic ministers or if someone in the group is musical, encourage them to sing or play an instrument. You can choose a Mass time and encourage the members to attend the Mass, then host a potluck dinner in the parish hall afterward for fellowship to round off the night.