You can actually do this pretty easily with the magic of variables, allow me to post a simple screen shot!
As you can see, I set the first variable to match the leader's level, you then add the new member to the party. The next part requires a bit of math, the first part you do is set the second variable to match the first variable, and the tricky part is, to make sure you are not ABOVE the leader's level, is you subtract the new party members current level from the variable.
Finally you increase the new party members level by the second variable, which will make the levels match.
I hope this made sense!