It’s fairly straightforward to add guest users to a MS365 group one-by-one in Outlook on the Web, however adding them as an admin is apparently a lot more difficult, especially if you need to add them in bulk.
Fortunately, there is a way to add them as guest users in AzureAD, and from there it’s straightforward to add them to a Microsoft 365 group.
Sign in to the Azure AD admin centre at https://portal.azure.com
Go to Azure Active Directory > Users
In Users, there’s a Bulk operations button at the top of the list. We want to Bulk invite our list of guests.
Download the template CSV file and edit it to suit.
I don’t actually know what the Redirection URL is used for, but it’s required so I just put in the company’s homepage – e.g. https://example.com.au
Most importantly however, I filled the Send invitation message [sendEmail] column to FALSE so that all my new users would not get a welcome email. Last, but not least, I put in a vaguely suitable welcome message, just in case they somehow did get sent an email.
I then uploaded this in to the Azure AD admin centre and it took a couple of minutes to create 50 or so guest users. Once they had all been created, I could then select them to be added to a Microsoft 365 group.