Hey,
the 1st step would be to add that guest user in your Azure AD.
https://datasharkx.wordpress.com/2021/05/13/inviting-guest-user-accounts-with-no-mail-enabled-to-an-azure-active-directory/
Once the guest user is present in the AD, then you can either add the user as Analysis service admins (highest privilege's) or create and assign roles as needed
https://learn.microsoft.com/en-us/azure/analysis-services/analysis-services-database-users