Trouble directly adding members to a Google group
If you use Google Workspace (née G Suite, Google Apps [for your Domain]) you may have come across an issue where new members cannot be added directly to a group in Google Group. You’re still able to invite but the directly add members option with be greyed out. This applies both to internal and external members.
This happens when the group identification requirements cannot be met by the potential new members. Typically this will be set to ‘organisation profile only’ meaning members will use their Google Workspace Account name, as shown in the directory.
External members don’t exist in the directory though, so don’t have an organisation profile. If allow external members has been enabled then you won’t be able to directly add any members from groups.google.com, only invite them. You can still directly add members, internal or external from the admin centre.
To fix you can either disable allow external members (not usually an option, assuming it’s on because you have external members) or simply change identification requirements for new members under member privacy to ‘display either the name or organisation profile’ or ‘display profile name only’.
Naturally the linked support article doesn’t mention this.