The system works pretty much that it lump sums everyone together in a party towards contribution. Meaning, if you have 8 people in a party, each person will get their own contribution + the contributions of all party members. It's done this way on purpose so that Healers and Tanks are also getting their fair share of contribution, since the best way to measure how much a person contributes is by DPS dealt.

This does mean that if youre not in a party, youre at a disadvantage because compared to full parties, youre getting 1/8th the contribution. Sucks to suck, but the system is the best one we got at the moment. You choosing to say "No, I dont want to hop into a party for this open world boss" is going to have concequences. I mean, even in a pragmatic sense, there is no reason NOT to party as it is impossible to solo these bosses, and there are no loot rewards awarded to players at an individual level. There is no individual player competition for world bosses, short of ensuring you get gold. So there is no real reason to NOT party and choosing not to is completely on you. Youre hamstringing yourself.