First date of first membership?
There may be a smarter way to do this in Views, but the short version is: I would create a View that shows memberships, then create another view that shows contacts and in the latter pull in the former using Views Field Views (ie you can say, show me the results of the first view based on a match for the contact id in the second view. and set it to show only a single result per contact sorted by oldest first, and that 'should' get you what you want. Hopefully I haven't missed out too many critical steps.