Forum OpenACS Q&A: how to tell, if a user is member of particular group

sorry for posting questions that perhaps might be answered in the documentation but i'm a bit frustrated with the latter and just need to get a few (simple) things done for my new site, so i haven't got the time and nerve to delve into the code...

having said that, here's my question:

how can i tell, if a user is a member of a particular group.

i already learned how it works to identify a particular user from the index.adp/tcl:

set user_id [ad_get_user_id]

and then

<if @user_id@ eq xxx>

but how do i do this for entire groups?

any pointers would be greatly appreciated,

thanks,

tom