Modifying members of Outlook groups

I have hit the problem myself and it is erratic as sometimes you can edit the membership of a group and sometimes you cannot. The problem is, I think, deep in Outlook and is linked to whether the client is connected to a Domain Controller of the domain owning the group (something taken from the depths of my mind). If the client is connected to a DC, you can amend the group because the group is owned by the domain and the DC has write access to the group. If not, you cannot. I believe that this is what happens with on-premises Exchange and it is reasonably logical. It's also the reason why people have advised for years not to depend on Outlook to maintain group membership (DLs). Now, Exchange Online is a different beast and Office 365 Groups are not DLs, but a fair chance exists that the same principles apply...

TR