By default, private Matters are only accessible by the Matter owner and those individuals above them in the owner's role hierarchy. AdvoLogix Participant-based Matter community sharing helps simplify the process of sharing Matters based on the participation in a given Matter.
Matter Community Sharing
This functionality can be enabled from AdvoLogix Setup.
AdvoLogix Participant-based Matter community sharing can only be enabled when the External Users Access has been designated as private for the Matter object . This can be done in Setup, under Sharing Settings by the system administrator.