Assign civi group by membership subscription?
in principe you could do that with the CiviRules extension (which you can get from https://civicrm.org/extensions/civirules). With this extension you can set up a trigger 'new membership' (or possibly a change of membership status?), add a condition for the specific membership type if required and add an action to automatically add the contact to a group.
There is a cookbook on CiviRules on the Wiki documentation: http://wiki.civicrm.org/confluence/display/CRMDOC/CiviRules+Cookbook