If you have your own site, dedicate one part of the site to "Attendance" where members of the group must post their user and chara name on specific threads, stating they were online.
I was in a roleplay where the group staff had family nights and game nights where we oocly and icly played truth or dare, hide and seek, tag etc. It was so much fun and it kept people wanting to get on, sadly that rp died. Though the family and game nights were a good idea.