"I have selected performer as someone from group..."
Is there no option for "all users from group"?
Ah right - so what you need to do is manually compile the list of performer (some of users in group) and check if each of these performers have completed the task. We did a long time ago by adding custom repeating attributes on case folder (or document) called reviewers and review_date and then after the workflow performers select reviewers, we created a process data mapper to copy values to object. Then after the review activity, create custom java module activity and assign date to review_date for the reviewer and then check if all the review_date values have been filled out. Only when this criterion has been accomplished, can the custom java module activity proceed to the next task in WF.
This design deficiency has since be addressed in xCP.