Two things might help your friends:
Clearing their Cache
Region Restart
And, have they tried TPing to your group SIM from a different location? Here's a "Troubleshooting Teleporting" knowledge base WIKI link for you. It includes a number of suggestions, including taking off attachments -- But, if your friends only have trouble to your group SIM, try the 2 options I listed above first.
http://wiki.secondlife.com/wiki/Troubleshooting_teleporting
Hope this helps.