LOTRO has maximum group sizes. Some Raids have max 12, others have max 24, etc. I also may use RaidPlanner for GuildWars and/or GuildWars 2; GW also has a maximum group size, though I’m not sure about GW2 yet.
As a suggestion for a future version, add a max size setting that can be set at Raid creation, so everyone can see if a raid is already full or not.
The numbers of Attending ppl. are displayed on the upper right corner. If RP locks the raid once the maximum size reached, and later someone changes his/her status you can lose attendants. Also, if there are different roles/classes needed for a raid, it can be problematic (imagine, when 12 healers are registered to a raid, and neither tank, nor dps can register after)
Raid leader has the possibility to confirm who will be in the actual raid.