[web] Update AddUsersList to call useUserSearchIndex

Summary:
There are several search experiences in the app that do not use get/usePotentialMemberItems. This means we need to update them individually, so I'm separating out diffs to do exactly this to make it easy for reviewers. [[ https://linear.app/comm/issue/ENG-6019/[after-landing]-try-to-update-remaining-search-experiences-to-use | ENG-6019 ]] tracks attempting to update these search experiences to use usePotentialMemberItems. Each diff will affect one file ideally.

This diff affects AddUsersList.

This component is used when adding friends from friend list on web

Addresses ENG-5434

Depends on D10252

Test Plan:
Please see the video below where I add friends from the friend list on web

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10253