I didn't see a guide on this, so I started this one. It's a work in progress because I'm not very good at griefing people. I solo too much, so I do not get a lot of practice. For now, this will just be an incomplete list of things you can do to grief people.
I'm going to leave out things like "abuse voice chat" or "rip people off in trades"
Category One: Share Please
1) when someone joins a group and says share please, share the quest "Emerald Claw Nugget" to destroy their Silver Flame Amulet
2) join IP groups and attempt to share every quest in your journal (be sure to step into a quest/wilderness zone first). Bonus points if you get someone killed
Category Two: Blind Invites
1) when given a blind invite, accept, then step into an unrelated quest (or, step into the actual quest and spawn 4 level 1 gold seal hirelings)
2) when you see someone soloing content, spam party invites at them. Again, bonus points if they die.
Category Three: Spells
1) cast Grease or Sleet Storm in any quest
2) cast Grease in key locations (bridge in Tower, entrance ramp in Sub, etc).
3) Greater Teleport people to the Twilight Forge
4) ...at the end of a vale walkup, after saying "gather on me for Teleport to Meridia"
5) Greater Dispel people
6) ...when they are meleeing Harry
Category Four: Quests
1) pull every lever you see
2) ...in Genesis Point
3) Choose the wrong NPC dialogue options
4) ...at the Litany dragon
5) run the quests Undermine and Blown to Bits
That should be enough to get things started.