[lib] Avoid repeating logic in getThreadOtherUsers
[lib] Avoid repeating logic in getThreadOtherUsers

Summary: While refactoring something else I realized this logic was repeated in a couple places.

Test Plan: Flow

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1784

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