Kinto, I'm pretty sure you can just show the message that you want to be displayed after the character is added to your party in the events... :/
Link, before you go any further with RPG Maker, I recommend you learn some of the basics, i.e. switches and (if you haven't already) variables.
However, you can get what you're trying to achieve without using any switches. In the character's event, stick a second page and under the "requirements" section (I can't remember exactly what it's called, it's on the left hand side of the events window), tick the box that checks for a character in the player's party and select the character you're dealing with. On that page, make the graphics blank and leave the events blank. Make sure it's set to "Under Hero", too, so that the player doesn't find an invisible wall there.
You can do the same thing with switches; on the previous page, after all the events have happened, turn a switch on. Then, make a new page with a blank sprite and no events, or even a different sprite/set of events, and simply find the box that checks whether a switch is on and select the switch you're working with.
Hope that helps.