To be able to target your messages in your signatures, you can assign campaigns to certain users or user groups.
In the Edit menu of your campaign, you have an Assign tab:
Here you can choose the exact list of users that will be assigned to your campaign. Either by group (2) or by user (3).
Don't forget to save the modification at the end!
Tip: You can track the exact number of users assigned to each of your campaigns from the campaign menu (list format):