I manage a French Sales Best Practices group since 2 years, with 2500 members ...

During the same time, I've launched the english group both on Linkedin, Plaxo and Facebook

Linkedin : 235 members http://www.linkedin.com/e/gis/35771/4260E2410212
Facebook : 4 members http://www.facebook.com/group.php?gid=5140149153
Plaxo : 255 members http://pulse.plaxo.com/pulse/groups/show/64426115524/
(Plaxo is just a service for direct connections and communication between members, and some of your 350 Xing members have join us.)

I've received 300 Linkedin invitations since the group creation,
200 from Plaxo Pluse.

So my tips is ...
Join Plaxo Pluse !
Plaxo allows us to bypass the Linkedin limitations

Best Regards,
--
Laurent J.V. Dubois
Your French Sales Partner
http://www.linkedin.com/in/frenchsalesconsultant