Users can only be added to Groups by System Administrators or those users with additional permissions to Manage Users. Once users are added to a group, they immediately have access to any Apps, Workspaces or content shared with that group.
Go to the Settings on the top right of the menu.
Select Groups from the Settings menu. You will see all current Groups in the system.
Click the Group to which you need to add the user(s). This will open up a panel where you can see all the users currently in the group.
Click the Add Users button on the right hand side.
This will open a pop-up where you can find the user(s) you wish to add. Only users not already in the group will display in the dropdown.
When you have finished selecting user(s) click Save.
