Using Classic Outlook on Windows for personal email, calendar, and contact management
Not the OP, but I keep receiving the same error of "Manage members failed for 1 user(s):."
When trying to add through Exchange Admin Center, I receive the following:
"An Azure Active Directory call was made to keep object in sync between Azure Active Directory and Exchange Online.
However, it failed. Detailed error message: Resource 'bc4e7d66-aec4-4caa-a7db-4b8a290e24da' does not exist or one of its queried reference-property objects are not present.
RequestId : f5185c3c-dbd3-41c9-9e49-83cf7fba135f
The issue may be transient and please retry a couple of minutes later.
If issue persists, please see exception members for more information."
I have waited several minutes and logged out then back in.
Edit: This is for an external contact that was first created as a Mail Contact.
Issue resolved itself.
After nearly 5 hours, the distribution groups allowed the members to be added.
Just needed to sync, but took a very long time.