When creating a new client or lead, there is a field called "Referral Source". This field contains a list of options which are also seen on the Collums' lead form on your website (if you are using it).
Create your own list of referral sources
Go to Admin>Lists>Client referral sources
Enter the options in the order you wish to appear in the list
Save
The option "Friend" is already in your list. If this option is chosen, the "Referred by" option becomes active. This then allows you to select an active client on your database to record who it was that referred your new client.