I have a membership form on one of the sites I manage. I use the WP Contacts plugin to integrate a Contact Form 7 form with a WP Contacts database. At the bottom of the form you just create a field map of the CF7 form fields to the WP Contacts database fields. It's well documented in the WP Contacts docs. Actually, I've done this with several CF7 forms. User submits CF7 form, data are collected into a new record in WPContacts database. Data from WP contacts can be selectively downloaded to CSV or Excel. -> wpcontacts.co/