Using older versions of Outlook for Mac in business environments
Hi, Ruth. I am an independent advisor, trying to help other users in the Community with my experience in Microsoft products. Please note that I am a user like you and don't work directly for Microsoft.
I am sorry you are experiencing this problem. I apologize for any inconvenience this must be causing you.
Please ensure the members you are trying to add, are users of Microsoft 365.
Please ensure you are currently using the updated version of Outlook. You can check for updates by going to File > Office Account > Update Options > Update Now.
I would also recommend using the Outlook web app, instead of the Outlook desktop app to manage your group. please visit Outlook.com and sign into your account.
I hope this helps you. Let me know if this contradicts what you need. I remain helpful in answering any more of your questions.
Warm Regards
Femi