You can add members to a group using the “ADD MEMBERS” button and search box on the members tab of the groups page. (Remember to first select the appropriate group in the groups panel.) Search for and select as many users as you like to build a list, then click “ADD MEMBERS”. A window will open asking you to confirm the action and indicating which groups these users will be added to. 

NB Adding users to a child group will automatically add them to the parent groups above it - these are indicated on screen. Learn more about group hierarchies and membership.

Users can be in as many groups as necessary.