@Mathias Sundman Thank you for reaching out to us, As I understand you are leveraging API-driven inbound user provisioning to on-prem Active Directory, though users are getting created successfully however you are looking for an option to add these created users into groups automatically.
As far I am aware this feature ( of adding users to group) via API is not available, assuming you might need to have some PowerShell script to do this job when the users get created in on-premise AD via API driven provisioning.
However I will check with my team internally on this ask and keep you updated.