I appreciate I could create an AD Group for this but it adds a delay in the site owner requesting the helpdesk to action a change.
I have site collection A and site collection B.
In site collection A I have a group called UserGroupA and in site collection B I have a group called UserGroupB.
The members in UserGroupB need to be the same as those in UserGroupA.
UserGroupA is the master and changes to it to either add or remove a user needs to workflow the change to UserGroupB.
Easy peasy I'm sure
we have implemented such a solution.
With a simple list for the group our users can add people with a People Picker field
and then an automatically starting WF adds the user to the AD group.
Users need no special permissions in AD this way.
Maybe this is a possible solution for you too?
create a scheduled siteworkflow, which compares UserGroupA and UserGroupB on the Sitecollections.
If there are any changes, delete all members from UserGroupB and copy the Users from UserGroupA to UserGroupB.
Or you create a siteworkflow which deletes every day UserGroupB and copy the Users from UserGroupA to UserGroupB
If you need a immediately sync of permissions changes you should try the solution of Rouven.
or another idea: if you can reorganize your sharepoint structure, you can create one Sitecollection with 2 Subsites.
I hope my ideas will help you.
I agree with Rouven Bilgeri about creating a list which will trigger workflow to manage your both user groups, no mater are they AD or SharePoint groups it will be just different workflow approach's of doing that.