Recently Returned to the Site/ Rusty and Need Input
From the GM Menu of your game, under Players & Characters, select Edit Players. From there, you will see buttons that allow for editing existing players and for adding new ones. There is the "new" method, which uses a Wizard to walk you through adding a player, and the "old one-stop" method that displays all of a player's details at once. Both methods have their uses.
Try these out, and, if you have further questions, jot back.
If you mean to invite people who are not members of RPoL, they will have to register and get a user account (there can be only one account per person!). If the game is Adult, it will take some time for a new user's adult access to be verified and approved. (A moderator can give a better view of how much time it will take.)
After they sign up and get their accounts, your friends should reply to your game ad, requesting to join (RTJ). At that point, you, as GM, can add them to the game as already described above.