If you login to your cnMaestro account and go to services tab and into Guest portal and select your Guest portal which you have configured to use for social login. Once you are there go into Access Control tab where you have entered your social login ID for your google or facebook app, if you notice there is 'Domain URL' which is listed there and you need to configure this domain URL in your facbook or google accounts. Facebook and google both check for origin URL's corresponding to your account ID so nobody else can use your account ID from any other webserver. Without this it when you try to login from guest portal login page it will throw an error of mismatched origin URL.
About your other question where if user doesn't have social account then you can also enable voucher policy and create and distribute vouchers to your clients.
Under the same Guest Access Portal->Access Control you will find Vouchers config. You can enable this and and create a plan for these vouchers. Once you have created the plan under Voucher then you can generate vouhers. These vouchers can be exported in PDF/CSV form and you can print and distribute them. But if voucher creation and distribution is something which you think doesn't work for you then may be you can create another WLAN with a wpa2-psk encrypted wlan and pass on the keys to the end customers. Or if you still want to have the Captive portal solution for it then you can use a second WLAN configured for onboard Guest-Access and use 'Internal RADIUS Guest User' configuration and give this user-name/password to your end customers. The onboard Guest-Access has the option of putting your business page in background and logo so if you think this works best for you then you can use that too.