Click the green "Add New Player" button to add someone to the campaign. You can add a player from your friends list or by username, and you can set their admin roles at the same time (see 6.5.1). Click "Add Player to Campaign" to finish. If you'd rather invite someone who isn't on Chronica yet, use the invitation flow.

Only Game Masters (and the Game Owner) can add players.

Add new player button

Chronica new player screen