With Groups, you control permissions centrally for entire teams instead of individual people, and reflect your real department/team structures.
Create Groups
Settings → Workspace → User Management → Groups → "+ Create group" → name & description (ideally by department/team) → add members → Save.
Manage members
In the Group, open the "Members" tab → add/remove. Members can be individual users or entire Groups. Changes take effect immediately.
Change a user's rights
Settings → Workspace → User Management → Users → "Groups" tab → assign a Group with the permissions you want → save.
Set a Group's permissions: Permissions tab → Manage permissions → choose a standard role or a custom role.
Recommendation: Start simple; only use nested Groups when you really need them.
Delete a Group: via the three-dot menu; users only lose the rights granted by that Group.
How Groups, roles and permissions relate to each other and what the standard roles can do is described in the article "The three standard roles: Standard user, Key user, Admin".