There is a plugin in our software called "Signup affiliate automatically to campaign". This plugin automatically signs up affiliate to private campaigns( http://support.qualityunit.com/917106-Private-campaigns
) after the signup.
IDs of campaigns must be passed in a_cid input field via custom HTML signup form( https://support.qualityunit.com/791436-HTML-signup-form
). For signing up to multiple campaigns, use campaign IDs separated by comma.
Example: <input type="hidden" name="a_cid" value="campaignA,campaignB,campaignC">