Functionality available on a page becomes available to a new site when that page is added to its lineup. We list useful examples as we discover them.
Note: we speak of a lineup as if it belongs to a site, the origin site, especially when logged in and writing.
Borrowed Roster. I often maintain a roster for each site collecting collaborators. I can hard-transclude that into another site's roster but for temporary use, I can just drag the desired roster's page into the desired site's lineup and start enlarging the neighborhood there.