Activating a character

[Hope]

I'm in the Ancient Temple and have given all the parts to repair Gravitron to Norman. While I played M1, I am new to RPG games and try to use a walkthrough for help occasionally. It says to put Gravitron in my active party but I can't see any way to do that, or to deactivate a character for that matter. I've looked on their status and everywhere else but I can't figure it out. Can someone help me please?

[D-Squall]

Press A in your keyboard to open the Party Changer menu.

[Hope]

Thank you so much!!!