Admin can assign multiple groups to a single user. please keep in mind these steps

1. There is a 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 are 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.

Sub Groups of Group

We can create sub groups of any group. And sub group can be assigned to any user.
For Permission -
By default sub group will get all permissions of parent group. You can revoke/add any permission from/to any sub group.