CloudM automations, particularly Offboarding Workflows (including Archivals), Backup policies and Signatures, can be run through Organisational Units (OUs) and Smart Teams. To run an automation for a Google Group, you need to use a Smart Team.
A Smart Team is a Google Group with the extra settings CloudM needs to use it in automations. A standard Google Group cannot have an automation assigned to it, so convert the group to a Smart Team first. If you do not have a group yet, you can create a new Smart Team instead.
For more on how Smart Teams work and the different ways to create one, see Smart Teams Overview and Create a new Smart Team.