Is there an option to just add a credit card to a User with the Braintree drop in UI, or can it only be used for a payment? I have my server configured to send tokens and receive payment, but I would like my Users to be able to add a credit card before using my app.
No, You can't add credit card only.
As per my email communication with the Braintree support team, If you're using Drop-in Payment UI, then you can't add a credit card only. While doing any transaction only, users will be able to add a credit card.