How to create a Wordpress User Profile when creating a Membership with Civi?
You can require users (or give the option) to create a WordPress user account in the advanced settings in the settings of the profile used on the membership contribution page.

You will have to check the box 'Anyone can register' in the WordPress general settings. I would also recommend installing a recaptcha plugin for user signups to prevent a large number on user accounts being created by bots.