I have a query for clarification that I can't seem to find in the rules or the forums (could be I just couldn't find it!)

I've just started a new campaign with 2 rebel players, we've just completed the first side mission.

Since then, one of their friend's has expressed an interest in joining the campaign, yet none of us are overly keen to start again from scratch (the person in question has played a campaign with another group before).

To do this, would it be as simple as setting up our next session as if we had 3 rebel players all along - i.e. legendary/heroic items and personalized side missions? Or are there other balance issues we haven't thought of?

I would give start the new player off with xp equal to the amount gained by other heroes so far. Also, allow the new player one rebel upgrade stage.

There is nothing in the rules regarding this, but here are a few ways to overcome this:

Have the third player jump in with equal xp as your other heroes and play with the heroic awards and side mission like you suggested.

OR

Have the original players play as 2 characters each, and when a third or fourth player wants to join, they would simply take over one of those characters.

We had a similiar situation with 3 rebel players and 1 more joining after the first side mission. We gave him the same XP as the other rebels, adjusted the rebel money (so that they got credits from the story mission like the 4th rebel was there) and gave the rebels an upgrade step. Worked well in the end.

During the campaign we had few times when one player wasn't present. In those times, the other rebel players collectively decided what his character would do. And after he came back, he had a chance to spend his XP before the next mission.

Thanks for the suggestions (i completely forgot the upgrade stages, good call); we introduced the 3rd player without a hitch, it seems to of balanced quite well