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