[web] implement adding thread members in AddUsersModal

Summary:
ENG-8568

Updating component logic

Depends on D13198

Test Plan:

1. Created thread with users A and B.
2. On A add user C.
3. Make sure C is added on both A and B, and C has now able to see the thread.

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13199