How to copy users between groups
Instead of adding one user at a time into a SharePoint group, you can directly copy users from one SharePoint group to another across different site collections.
Steps to copy users between groups: Click on the Management tab.
Under Groups Management, select Copy or Move users between Groups.
Choose the site collections in which you wish to perform the action.
Select the group whose users you wish to copy in the Select Source Group field.
Select the group to which the users are to be copied in the Select Target Group field.
Select the users to be copied in the Select User field.
Select the Operation type as Copy Users.
Click Apply.