Heya 
To get your players started on The Forge, they’ll need to have their own Forge accounts. It’s possible to make a Forge account here
If they have Forge accounts and are instead not able to select Foundry users, you as GM would need to create some Foundry users for them as players to use in the game. For that, see a guide here Tutorial - Gamemaster Part Two | Foundry Virtual Tabletop
If you are on Story Teller tier on Forge, then you’re also able to activate Game Manager which lets them use their Forge accounts as Foundry users.
Hope that helps! 