As an admin of a Facebook group, can I see who leaves the group?
I'm not aware of a built-in tracking feature for that (I'm a group admin too). As an admin of a closed group, the member tools seem limited to accepting, removing and blocking members.
Looking at some of the site FAQs, I couldn't find a clear answer to your question, although there are a couple of pages that indirectly relate to member tracking, and both seem to suggest that the absence of tracking is by design, and may relate to Facebook account privacy provisions:
-- On the How do I leave a group? Facebook page, it states that when you leave a group, "members won't be notified" (although it's not clear whether they consider group admins equivalent to members in this answer).Source: Facebook - How do I leave a group?" - https://www.facebook.com/help/172944012764072
--The Bugs & Known Issues page mentions that group members whose Facebook accounts are inactive may continue to be counted as group members in the total numbers count, but no longer display their identities in the group (or anywhere else on Facebook). Source: Facebook - Bugs & Known Issues - https://www.facebook.com/help/261113790660525/