Please make viewing distribution groups members possible from the address book for Mac users. Please also make viewing what distribution groups a person is a part of possible.
There's a technical limitation here. Outlook for Mac doesn't connect to Exchange/Office 365 with MAPI/HTTPS (what the Windows client uses), it uses EWS, which has considerably less features available to it (one of which is getting details about Distribution Groups.)
So unless they decide to overhaul EWS to feature-parity the MAPI endpoints (but then what's the point of MAPI?), overhaul the Outlook app to use MAPI/HTTPS (not likely to happen any time soon), or have some EWS endpoints they have kept private all this time and are ready to go public with them, this request isn't very likely to happen.
And given how Microsoft is prioritizing almost literally everything else, (even trying to revive Yammer from the grave), over actually finishing development of the app (they should have flagged it as BETA or pre-release...), we're going to be waiting a long time.