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. You cannot retrieve the player password, it is encrypted, so if you forget what it is, you'll have to change it and notify your players of the change.
The league administrator can change passwords under the Administration -> League Info section.
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!