Only a landlord or lead tenant can add a guarantor to an application.
If you are the lead tenant
If you are the lead tenant, you can add the guarantor by simply entering their name, email address and phone number through the website.
The landlord will then get the guarantor's details and can add them to the application and order referencing at their discretion.
If you are not the lead tenant
If you are not the lead tenant, you should send the guarantor's name, email address and phone number to the landlord or lead tenant so they can add them to your application.
If you are the landlord
Go here for How landlords can begin referencing tenants and guarantors.