I encourage all new players to take a minute and look at this.

[Puck]

Is the 30% rule just when your solo and some one else jumps in or does it apply to parties also?

Just solo. In groups, everyone gets credit and full xp.

Btw, probably obvious but this goes for harvest nodes as well... If you see someone come up and start chopping on your tree, they're not taking anything from you.