I've been a linkedin user for almost two years now. I use it to maintain my resume online, and to keep in touch with people I work with or that I've worked with in the past. I also use it like a global profesionnal address book - so I don't clutter my phone nor my current address book with entries I'm not using. I've been looking at the other features offered by linkedin, you have question and answers, but I'm usually too late to give a proper answer, someone as probably replied before me. The other feature is that you can belong to groups, I'm a member of Fosdem attendee for instance. I got into the group after finding out that a friend was part of it - then I decided to see if I could fit in more groups and decided to look for groups.
There is no way I could search for groups in the linkedin interface (and if there is it very well hidden).
The only thing available is a group directory, but I can't even find the group I'm a member of in the directory - which seems to not display all groups.
I've tried serachnig for "group keyword site:linkedin.com" on google but was unable to find groups, the only thing I stumbled on was profiles.
Any idea how I could serch for groups on linkedin ?
so thanks to SvZ I got a more open listing at http://dallasblue.com/LinkedIn/groups.htm

You might want to try this:
http://www.ere.net/blogs/CyberSleuthing/0778ABED4B3E4C30A862493B166451AB.asp
HINT: How did Randy add all those groups? He right-clicked on each of the icons under the Additional Information section of a profile and selected "Open in new tab" or "Open in new window" then simply accepted the invitation to each of those groups :)
Got the same problem, can't find any groups, because it's actually disabled. According to:
http://blog.linkedin.com/blog/2007/09/linkedin-groups.html
"The groups directory is not currently available for new groups. If you have questions about LinkedIn Groups that are not answered in the FAQ, please contact us here."