Users of membership sites often have and use a membership number. Yet WordPress uses alpha numeric string for usernames for logging in. Why not use both by using the LH Membership Number plugin.

LH Membership Numbers allows a user to login by typing in their membership number. Their membership number is the unique user ID that every user is assigned in the database. Optionally administrators can assign a prefix to the membership number (eg for my sports league princesparktouch.com it is PPT). That prefix can make the number far more unique (globally).