Now admin can assign multiple groups to a single user. I am following these steps

1. There is column user_group_id in users table which holds comma separated group ids of user.
2. If any user has more than one group then the user will have all assigned group permissions.
for e.g. there is 2 groups A & B
A has permission to Edit User details
B has permission to Delete User
Now If user is assigned both groups A & B then user will have both Edit & Delete permissions.