Go to JIRA and vote! As of this posting this issue already has 687 votes. As stated by the op:
"Groups are far too useful and used to be limited to only 25. People talk of "cleaning out" their groups list to make room for a new one... Ridiculous. Between groups associated with online places (clubs, shops), social groups (my friends), cultural/interest groups (Llama fanciers of SL), business groups (vendor in mall XYZ), and the occasional land partnership it is very easy to max out your groups. Considering how critical groups are in managing stuff like businesses (who gets to build what where), there ought to be a way to increase the limit at least to one or two hundred."
I absolutely agree, as does NWN. We NEED more than 25 groups. Honestly, I don't see why there is such a small limit. Now go go go! vote!