How to customize the fields in Membership plugin's registration form

I'm using the Membership plugin. How can I add fields for First Name and Last Name to the registration form?

Right now, it just asks for Username, Email and Password.

We are a membership-based organization and only allow existing members to have access to a certain portion our website. It would be helpful to have their names included in their registration process so we could more quickly approve/deny users access

Thanks!