First, they must have TaleSpire (downloaded from Steam) and a valid key.
Second, to add new players go into TaleSpire and press the space bar. Then click on Campaign Settings and you can generate an invite code to give them.
Once they have the code they can use it at the login screen to join the campaign.
Then if you want them to help out, you can promote them to GM (if you need their help running the game).