It depends on the add-on. Once it's enabled on the document all the document editors will see on the Add-ons menu > Document add-on that there is one or more add-ons enabled on the document but the add-on might not have any custom menu shown to them.
To use an add-on one user should install (this is done by user) and enable the add-on on the document (or form, or slide or spreadsheet) (this is done by document). Custom menus might be dynamic and menu options might depend on user properties or other stuff.
NOTE: This is assuming that the questions is about editor add-ons. Workspace Add-ons have a different "lifecycle".
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