An add-on's custom functions will work for all users of a spreadsheet where the add-on has been enabled by any user. See Installed versus enabled.
To enable an add-on in a spreadsheet, run any function in the add-on. The function does not actually need to do anything. Executing any bit of code in the add-on will cause the authorization dialog box to show, and when you give the authorization, the add-on's status changes to enabled for that spreadsheet, and custom functions in the add-on will run in that spreadsheet for all users. Running a function is usually done through a custom menu item.
To disable an add-on, use the Extensions > Add-ons > Manage add-ons dialog box.