Gagan Deep Singh

Allows only logged in users to visit the site. If anyone tries to access any page on the frontend of the site, user will be redirected to the login page automatically.

For Users Only

Allows only logged in users to visit the site.

shawfactor

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 <a href="http://lhero.org/plugins/lh-membership-numbers/" rel="nofollow ugc">LH Membership Number plugin</a>.
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 <a href="https://princesparktouch.com/" rel="nofollow ugc">princesparktouch.com</a> it is PPT). That prefix can make the number far more unique (globally).

LH Membership Numbers

yesim the plugin author but this plugin works perfectly as described (it is a very simple plugin so it is not for everyone).

Don't be f-wit like bgtbbox (the last reviewer), read what it does and if you don't think it works a specified put something in the support forum and I'll look at it.

Works perfectly

Plugin to allow users to login by a number, their user ID (optionally prefixed)

For Users Only vs LH Membership Numbers