From the list of menu options at the top of the spreadsheet, select Add-ons, highlight Group Gator and click Manage Groups. See the diagram below for a visual.
Once you have selected Manage groups, you may get a prompt asking for permissions, as it needs to access multiple Google services on your behalf.
Once installation has completed, Group Gator will open as a sidebar on the right side of the screen, and is ready to begin managing your groups. Seeing something else here? This means your domain admin hasn’t properly installed Group Gator for your domain. Let them know!