I'm totally new to this - Party members and XP

[Chrisern]

If I choose to leave some party members behind in camp, will my active party members get more XP because there's fewer to split it between? Or how does this work?

[TamTroll]

the only time one party member will get more experience then another is any time they get an inspiration, which is something like 30 xp for them specifically. Those moments aren't super common though, so they won't matter too much. all other XP is shared.

Any time you pick up a new party member from camp, they'll automatically be given any xp they missed out on, and catch up to whatever level you're currently at.

[Metallicus]

All experience is split among all the companions. It is not possible to ‘fall behind’, but it is possible for a character to get bonus experience from background events which have a very minimal affect on leveling (typically less than 100 XP per character).

[♂Warrior of Gachi♂]

It doesn't matter, everyone will always have the same XP give or take 100-200.