We’ve just released a new version of the UserGroupShipment module for Commerce. Version 1.2.0-pl brings compatibility for MODX 3! 
Download and enable the module in the Commerce configuration tab, set up the target user group, and on your chosen delivery type set the shipment type to: Assign User to User Group. Finally, choose which products should use this kind of shipment.
Customers will then be added to a user group of your choosing on completing the checkout flow with that product. Fantastic for selling access to restricted sections of your website!