We had to temporarily take groups offline due to an import issue importing wrong privacy for private groups. i.e. private groups from Truong Groups were imported as closed groups exposing sensitive group content like financial data and personal photo's.
Circling back on this, can you try going to the Maintenance page and running the "Rebuild social group members" and see if that fixes the issue? It should already have ran post import but maybe it didn't finish correctly for whatever reason.