When a membership is approved via a membership application, the option to select a Join Reason on the application is not available. However, once the application is approved and the contact has a membership on their record, you can add the Join Reason for reporting purposes.
On the contact's Overview tab, click the name of the membership assigned to the contact.
In the Membership Details window, in the Membership Activity section, click the Membership Type Name on the line with the Activity Type set to "New".
In the Edit Membership Activation window, click the drop down arrow to select the Join Reason you wish to apply to the membership.
Click Done to save the existing membership's Join Reason.