To be able to use Introducers against your client records, you must first setup the Introducer within the Contacts module.
Click 'Add record'
Enter the details of the Introducer and once done, click the 'Introducer' tick box on the bottom right of the screen. As seen below:
Your Introducer can now be selected on your client record.
If you would like to track client activity and add remuneration information, select the 'Activity' tab in the left menu: