I think you do not change what a certain repository group permission does. You just select one of the groups for each user. Sometimes these permissions override role-based permissions. For example, a JazzProjectAdmins member can always save a project area, regardless their role permission settings. What a particular repository group can do is defined in the help below:

https://www.ibm.com/support/knowledgecenter/SSCP65_6.0.4/com.ibm.jazz.platform.doc/topics/c_permissions.html#c_permissions__jazz_project_admins

You DO change the role-based permissions, and this is highly customizable. (See the same help link above.) You can create new roles who can perform the actions that they need to.

If you are using WAS, I think you can assign different permissions to the repository groups within WAS, but I don't think this makes any difference in RTC.