This group just has user title and user name markup, to make it easy to spot when it's a member's birthday.
See this post for details: http://xenforo.com/community/thread...on-a-members-birthday.3170/page-2#post-274811
When the user logs in on their birthday and the User Group Promotions cron runs, they are added to the group.
However, after a period of inactivity, they are demoted and removed from the group, despite being logged in still; i.e. they are still showing in the Members Online Now list.
The next time the User Group Promotions cron runs they are added to the group again.
The cycle continues throughout the day.
I have confirmed that they are added to and removed from the user group by checking in the ACP.
It seems as if another cron task is demoting them, despite the criteria still being valid.
I have run all cron tasks manually to try and trigger the demotion, but I was unable to.
I've already discussed this with Mike & Kier and provided screenshots, etc. so this is just the official bug report.