You should learn how to use variables, they're very helpful. Tons of tutorials on lots of sites like this too, so you have no excuse not to learn. Anyway, what you need to do is assign a variable to do this.
To do this, go to "control variable" and pick one of the numbers. Name it something like "level setter." Now, the first time a character joins the party, you'll have to do the following:
Call this variable in control variables. When you go to control variables, you'll get a window that asks you to fill out 3 different things: Variable, Operation, and Operand.
Under variable, pick your variable that you named.
Under operation, select "+" (the addition sign)
And under operand, select "actor" then select one of the characters in the party in the drop down box, then select "EXP."
Keep on doing this for every character in the party at that time.
After all this, go to "control variable" again, but this time set the operation to "/" (division) and under operand select "constant" and set the number in that box to the amount of characters that are in the party at that moment. This is basic averaging.
Next, go to control variable yet again, but this time set operation to "-" (subtraction) and set the operand to 1. This will take that variable and -1 from the number you get after doing all this algebra. Next, have the character join the party, and increase his EXP by the variable (this is simply done by selecting the operand and making it the variable that you've been working with all this time).
And that's it. Of course, this version actually takes an average of EXP from all the characters. It'll do the same, but if you want the new character to start at a slight disadvantage, just make it so that when you select EXP on the first step, make it level.
Now go learn variables ^_^