Assigning marketing sets works very closely with the User group editing.
​We strongly suggest having the User groups defined before you get through to setting up specific marketing campaigns. 

After you prepared the marketing campaigns and organized them in Sets, you can assign those campaigns to specific User groups.

Please keep in mind that unless you choose otherwise, the subgroups will inherit the marketing Set assigned to the User group.

If you’re not sure how to insert marketing modules to your signatures, check the article about variables in signatures.