Returning members can simply log in with their existing username and password. If they've forgotten their login information, they can use the 'Forgot username or password' link in the login area. (Note: If you've deleted their membership in your pool, they can still login, but will need to rejoin your pool after logging in.)
New members can always join a pool you are reusing from a previous year. You can find instructions to send to these members under Commissioner Console --> Invite Members.