You can modify the groups where a particular user belongs through the User Details.
1. Go to Users and Groups.
2. Search and select a user from your user list.
3. Click on the edit button in the User Profile.
4. Scroll down to the bottom of the page to see the Groups. Click on Modify Groups.
5. Tick the checkbox of the group you want the user to be added to.
Click on Save once you're done.
6. To save the changes in the user's group/s, click on Save.
