I think that boosted exp should go to anyone who attacked the monster. This setup discourages from attacking somebody else's target. I know during the first few turns, I attacked a monster somebody else already wounded and I believe they were partially upset for me trying to take the kill (which I missed anyway). If we shared the boosted exp then we wouldnt have to worry about stealing kills and we could work more as a team.
I think that scyberdragon has a point here. We are aiming to be a team through this 'journey'. I know that I would be fine with this aim on the experience aspect. However I would adjust it to anyone who did damage to the monster rather than just attacking. The person who lands the final blow should still be the one to get the gold and special items.