My group is about to start a campaign for the first time using this app. However one person isn't going to be able to make a few sessions, particularly the first where we would pick characters. Is there any way to add or change the party composition after the game has started or are you locked in for the entire game?

Sadly no, this isn't possible in the app. Of course you could change the character and class the player uses, but the app is locked to your initial decision. So chose wisely

Oh well. We'll just have to make sure people pick well the first time. Thanks for the confirmation!