The following is a list of groups which are worth getting into. Some are self-enroll and for some you need an invitation.

EduNation residents
For residents of EduNation I and III and their students and everyone who needs rezzing rights on the islands.
Owner: Randall Renoir, Gwen Gwasi and Carolrb Roux, 160 members (June 2011), invitation only

EuroCALL in Second Life
EUROCALL is an established professional association devoted to the promotion of CALL (Computer Assisted Language Learning). It was set up with the aid of EU funding in 1993 and has over 300 members in Real Life: http://www.eurocall-languages.org This group aims to further EUROCALL's aims in Second Life. EUROCALL shares a location in SL with CALICO, its sister association in the USA:
http://slurl.com/secondlife/EduNation%20III/31/35/22
Owner: Groovy Winkler, 149 members (June 2011), open enrollment

SLexperiments Language Teachers
A group for Language teacher who teach or want to teach languages in Second Life. We meet in-world twice a month to share what we know, teach each other SL skills, demonstrate tools and go on field trips. We also have a wiki where we collect resources.
You can find more information here: http://slexperiments.pbworks.com
The meeting are every first Friday of the month, 6pm GMT on EduNation and every third Wednesday of the month, 7pm GMT on AVALON Learning.
Owner: Daffodil Fargis, 137 members (June 2011), open enrollment

"Ich bin ein Berliner"
For those who want to learn German. The group meets twice a week on Thursdays and Fridays at 12pm SL Time.
Owner: Gwen Gwasi, 54 members (June 2011), open enrollment

SLanguages Annual Symposium
This group is active once a year only at the time of the SLanguage conference, upcoming 16-18 September 2011.
Owner: Gavin Dudeney, Gwared Morgwain and Gwen Gwasi, 321 members (June 2011), open enrollment

Webheads
The Webheads are an open, vibrant and global online community of educators who are interested in the application of educational technology, and who meet virtually on a regular basis to share their findings and friendship. Originally founded as a group for English language teachers, we now have a wide variety of educators from all disciplines.
Owner: Webhead Link, 345 members (June 2011), open enrollment