[native] Introduce addSelectedUsersToCommunity to CommunityCreationMembers

Summary:
Introduce addSelectedUsersToCommunity which adds selected users to the newly created community when the user hits "Done." In next diff we will navigate to the newly created community after successful creation.

Depends on D8122

Test Plan: Check community and ensure that users were added as expected.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8129