manage several briefing meetings over a day

I have to organize several hour long briefing meetings during a day. Staff
can choose which time slot they wish to attend but I need to limit the number
of attendees to each session. Is there a way I can do this automatically in
Outlook when I send the message to all staff.

I can't send the message as a meeting request and use the voting buttons
then which would automatically put the meeting into staff calendars.

Thanks
Marlies