The operation cannot be complete. The term store may be unavailable" in term store management tool

hi all,

I've created a new managed metadata service in SharePoint. Then, i created a new term set. I've assigned the permission of Group Managers to my  service account[managed acount].

 I also want to add one of my SharePoint group in the Contributors field. But, it throws error "The operation cannot be complete. The term store may be unavailable".

Then, I tried to add users instead of groups. It's working for users and  AD groups only. 

Can you please let me know how can i add sharepoint group(s) ? i think MMS is still not fully baked. its a  bug ,it was appearing sp 2010 as well. here in sp2013 it repeated again!