Just to back up what Tirion said, heres what i posted in another thread "worrying" about a similar topic:

The reduction in bonuses from Linking and the Addition of finish bonuses will still make leve-partying a viable option until they add a load of content. The finishing bonuses with leves are better when you do it hard and fast (mind the pun).

So solo you can do say a R20 leve solo at 1 star in 20mins and you get 1500sp bonus at the end, but you try in a party that could do it at 4stars in 10mins and the bonus is much better and everyone in the party gets it.

Not actually sure about if everyone gets the same amount or if helping nets you just a little less than the "host" unless you link).