What engine are you using? This needs to be moved to the proper support forum.
Generally though, you would set the ally up in the database but not include them in the starting party. At whatever point you want them to join you, you would use the event command to add them to the party.
If you want it to be an instance of the main character talks to them, then they join the party, you would need to make an event with that ally's graphic and run through whatever dialogue you want. Then you would need to make a second page of that event, usually triggered by a self-switch, with no graphic image. Turn on the self-switch so the event ally disappears and use the event command to add the ally to the party.
Still alive! Totally did not finish in time for the Jam (boy that's late) but tl;dr of the last half a year is Work term -> Covid Lockdown -> Sick... again -> Full time Employment. Game stuff has been on hiatus but got a new graphics tablet so hopefully updates again soon. =D