In the list of event commands, on the first page, in the Party grouping, is the command "Change Party Member". Click that, and it'll give you a box containing a drop-down menu, a pair of radials, and a checkbox. Select the actor you want to add from the drop-down, select add from the radial options, and check initialize if you want the person to have the stats listed in the database. If you've had him in the party for a while, removed him, and are getting the guy back, you probably don't want to initialize him (unless he's got amnesia or something.)