User limitation

Hi,

has anybody done something to limit the number of users to a certain class
(or group)? denying authentication if a limit of users to the desired class
is exceeded? 

I would like to guarantee some minimal quality rather than having 100 users
at the same time crawling along...
does anybody have any ideas or ready to use code for this?

btw: for me htb works a lot better than did cbq traffic shaping

regards 
jj