League Player vs. Event Flex Player
When signing up players for your League Event you should always default to signing up the Player as a League Player. If they do not exist in your League and there is a possibility that they may play in another League Event you should add them as a Member or Non-Member to your League.
Reference this article for adding League members League Player (Add)
A Flex player is a Player Signed up in name only, they will not be able to Sign into your League and none off the statistics from their play will tracked against a Profile in Unknown Golf.
Reference this article for additional information on Flex Players League Event Flex Player