When you created your account with Golf League Tracker you created a League ID, and Administration password, and a Player Password. Everyone uses the same League ID to log in.
Administrators will use the administration password. If you can't remember what that is, click the "forgot password" link on the login page to reset it. If you have more than 1 administrator, they will all use the same league id and password to log in.
For players to log in, they will use the League ID and the Player Password the adminstrator set. If you're the administrator and can't remember what your player password is, you can change it in the "League Info" section in administration for your league. Just note that if you have already sent your players the password, you will have to send the new password to all your players if you change it.
There's is no specific "invitation" function in Golf League Tracker, but you can simply send a group e-mail by clicking on "Players" in the navigation bar and choosing "Send Group E-mail" and relay the appropriate information to your players. Be sure to send them the PLAYER PASSWORD!!