asterisk users - May 2006 - SetGroup and CheckGroup. Need some help on the dialplan

>From this list I found that I could use SetGroup and CheckGroup to do
what I wanted. But I'm not quite sure how I do it.

The case is that I have 3 user groups, and one main group. The main
group is for all the incoming calls from external phones. The main group
should be allowed to have 3 calls at the time. 

The 3 user groups are internal groups that I want to limit by ony having
one incoming call at the time. So if userA-Phone1 is on the phone.
UserB-phone 2 shouldn't get any calls.

Illustration: (hope it don't get messed up)
Incomming Call --> My Company --> Group1
						 --> UserA-phone1
						 --> UserA-phone2
                              --> Group2
						---> UserB-phone3
						---> UserB-phone4	
   					 --> Group3
						---> UserC-phone5
						---> UserC-phone6

So what I want per group level is that only one user (phone) is active
at the time. And if all of the groups are busy, I want to send the
caller to voicemail.  

Everyone can call out at the same time, but it must update the group
count the phone belongs to. 

The main problem for me is the dialplan that decides witch Groups are
available. And also how to decide witch group to update on outgoing
calls. (all the users use the same extensions and peer for outgoing)

Hope that made sense.

Thanks

Regards,
Arne Morten