Please remember that a user can be a member of multiple groups.
To assign users to newly created groups
Click on the group name.
Go to the Members section inside the Details panel.
Click on the + (Add) icon.
Select a user by clicking on its name or select multiple users by holding ⌃ CRTL
+ Left Mouse button, then click the Select button.
Repeat the same process for assigning multiple users to different user groups.