Hello Liz, 

Just a thought, Can you add both the user groups to the Outsystems application you created in Azure AD. Recently we had some external users added as Guest to the enterprise AD account to provide access to our application. This way you don't need to do any change in Outsystems. 

https://docs.microsoft.com/en-us/azure/active-directory/manage-apps/assign-user-or-group-access-portal#assign-users-or-groups-to-an-app-via-the-azure-portal

If that's not possible, How about a multi tenant app on Azure AD and configuring IDP with that application and metadata.

Kindly let me know which solution you go with , quite curious :-)