Introductions

Have the player stop at the Introductions part of their playbook. Decide together how you'll bring them into the game. There are two basic approaches:

Play it out. Introduce the new PC in play. Frame a scene with them arriving in town, or have them be there when the PCs get home from an adventure. Or, like, have that clumsy kid get struck by lightning during a crisis and become the Stormmarked Heavy just in time to save the day. Skip the formal Introductions on their playbook, but ask questions as needed. This approach can be fun and visceral, but it can also take up a lot of game-time and inadvertently set up PC-PC conflicts.

Time skip. The new PC appears (or steps up) between sessions, during a stretch of downtime. Establish that they've been here (or been active) a while. Have the new PC do Introductions more or less normally at the start of the next session (or between sessions, if that works for your group). Maybe write a love letter to each existing PC that both covers events during the time skip and that establishes their connection to this new PC.