You could be getting this error due to network or transient issues. I am able to invite the user with or without group members assignment from Azure and Azure Admin portal. Can you please check the documentation if you have any organization policies which is preventing you from inviting any user.
Adding guests to group error
David Johns
1
Reputation point
As the bulk guest invite function is currently disable i am having to add new guests individually. However i cant add them to a group at either the invite stage or afterwards within the group membership preview pages as i just repeatedly get the same error:
Failed to add group membership
Unable to complete due to service connection error. Please try again later.
Any advice or solutions welcome