Hi
I have found a good reference to your problem. Please have a look at the same.
Reference :- http://www.hsufengko.com/home/copy-users-groups-permissions-acls-between-adobe-cq-instances
// It covers copy users, groups, permissions (ACL) between Adobe CQ instances.
More References (less important, but good read)
Link:- http://stackoverflow.com/questions/24919675/cq5-aem-change-permissions-on-groups-with-curl
Link:- http://www.wemblog.com/2012/03/how-to-do-user-management-using-post.html
Your can also automate things as mentioned by Bsloki or can do things using Curl (Link:- http://www.wemblog.com/2012/03/how-to-do-user-management-using-post.html)
I hope this would help you.
Thanks and Regards
Kautuk Sahni