The player you wish to add must have an account on Chronica. From your user dashboard, click the name of your Campaign to access that campaign's data. Navigate to Party Info in the left navigation and click the green "New Player" button in the top right. Enter their account email, select player permissions if applicable (you can change these later), and click the blue "Add Player to Campaign" button. Game Masters have the ability to remove players, or players can leave campaigns when they wish.