So, first you must know the exact name of the Group in which you are interested.
If you are already a member of any YAHOO! Club, first go to that group. (Or, join any innocuous group.) Then, using cut & paste, or just type into the portion of the address window of your browser the name of the new club to which you wish to go. That will get you there, then you can follow the usual enrollment procedures. Let's say for instance, that I'm a member of a YAHOO! stamp collector's club and its address looks like this:
http://clubs.yahoo.com/group/stamps
I would go there, and then while that URL is visible in the address window of the browser screen I would substitute " womeninfullcutpanties " in place of " stamps "
http://clubs.yahoo.com/group/womeninfullcutnylonbriefs2
Go there and there you are! Then just join.