Only the Host take XP/rank after raiding
This is true for PlayStation co-op as well.
PC also, pretty sure it's intended.
it prevents me from using my level 100 boosts to hunt dungeons and carry newer players to the top.
I wasn't able to use my own boost, but I carried my friend to Suits level 50 so his boost would be worth it plus earning Cells to upgrade all his stuff, and while doing that I carried him from Bronze I to Master. The current system doesn't prevent carrying, it just sucks for the player who's not the host. The non-host goes through the dungeon while the friend with no upgrades stands around waiting, coming through when it's cleared out to get the much needed resources, and avoiding dying so they get the full amount of rank. As the non-host I feel like kind of a chump for playing to my full potential and not dying, I don't know why I'm bothering when it makes no difference whatsoever, especially when I'm mostly playing solo and I'd rather run the base with him since we're playing co-op but neither of us want him dying and losing rank. The system sucks, and it'd be nice if the non-host at least got half of the normal XP/rank.
Edit: Also, if this is intentional, then the devs need to make a statement and this information needs to be included in the Tutorial/Game Tips section, because it confuses players and leaves them feeling cheated. Reports of this as a bug show up constantly on the different forums and on Twitter.
it doesn't prevent carry, but it means you're carrying blind until they level up sufficently