Hello Matt,
The Get-AdmGroup cmdlet works fine with Azure AD groups when the corresponding domain in registered in Adaxes. Make sure that you specify the parameters correctly. Also, in such cases, the -AdaxesService and -Server parameters are mandatory. Finally, you should have something like the following:
Get-AdmGroup "CN=My Group\0AUID:a5bd42983e9d4947a866cdd1ee43ed29,OU=Groups,DC=azure,DC=onmicrosoft,DC=com" -AdaxesService localhost -Server "azure.onmicrosoft.com"