New to RPG Maker? This is simple event cutscene.
Basically set two events that represent the 2 characters not yet in the party. Use another event in autorun that makes the player walk toward the 2 character events (use the "Set Movement Route" command on the second page). Do the message part (I assume you know how to do that). After that, add the 2 characters to the party ("Change Party Member" event command, first page). You can then activate a switch to make the 2 character events disappear if you want (set a second page in those events with that switch as a condition and make it autorun. The only thing needed in that page is the "Erase Event" command).
