Experience bonus based on players in game:
 

Team bonus:

The bonus is shared using a complicated formula which basically gives more experience to the higher leveled characters in the party, and less to the lower leveled characters...thus avoids "rushing".

(CharacterLevel+9)^2/Number of TeamMembers((CharacterLevel+9)^2)

Explanation of formula...

Monsters types: (by circle colors under enemies)

♦ Grey: 4% of normal.
♦ Green: less than normal
♦ Yellow: normal
♦ Orange: normal
♦ Red: normal +10%

Note: Survival Bonus is used to calculate monster level, the higher your SB, the higher the monster level can be.

Other factors that effect experience:

♦ Items with + Exp
♦ Complete Sets (each complete Set gives +10%)
♦ Experience Statues (the big statues you click on)
♦ Mentor Potions (each potion gives +50%, non-stackable)
♦ Sunglasses- if one party member wears the sunglasses, the entire team is "cool" and gets a 7% bonus.

Note: the Sunglasses (quest reward from Mal-Orc-A) are considered head gear, so you'll break any set that requires headgear to wear them. However, your teammates will appreciate it!

Game Difficulty Bonus:

Exp. Penalty for sandbagging:

Penalty dealt if you're not leveling in the highest difficulty you can enter:

(maxDiff+1-hlvl)/(MaxDiff-NextDiff+9)

Explanation of formula...

Penalty for sandbagging starts at:

♦Level 12 in Bronze.
♦Level 53 in Silver.
♦Level 93 in Gold.
♦Level 133 in Platinum
♦Level 169 in Niobium.

Until then, you get a braveness bonus.

Note: You still may earn more exp. in a Bronze game with 5 players than you'll get in Silver game playing alone. Test it by facing the same monster types in the same area in Bronze and Silver.

This formula means that the higher your character level is, the lower the exp. gained will be as you approach the limit.

World Explored:

Effects exp. gain, the bigger percentage explored the more exp. you will get. Particularly important for high level characters who need to increase their exp. gains everyway possible given the character level exp. reductions.

NOTE: You can't explore it all. Much of the land mass cannot be reached, but try to get as much as you can.

Summary:

You will level faster if you fight alone against red monsters.

Enter new difficulty levels as soon as possible... BUT only when you observe the exp. gain in the two different games. That is: the bonus from a game full of other players might outweigh the penalty for sandbagging (overstaying in a difficulty level).

Explore the map. Your high level (>100) character needs a high map explored score to level better.

For fast exp.: maximize the number of monsters you meet... providing your character has the killing speed required to handle them. People tend to use the caves (or reverse caves... that is, in caves Horses are not allowed, outside the caves, horses are allowed. Thus, some players will want to play in caves (Vampiress) and others won't (Mounted Seraphim). Caves allow the player to control the respawn rate:

1) Orc Cave (south east of Khorad-Nur portal)
2) Ghost Cave (south west of Valley of Zhurag-Nar portal --across the river)
3) Frost Giant Cave (west and then north of Monastery of the Seraphim portal)
4) Undead cave 1 (just north of Faeries Crossing)
5) Undead cave 2 (Tyr-Fasul on the path to the fortress (south west))
6) Undead cave 3 (west of Khorad-Nur)
7) Giant Spiders (in MP use the portal room and take "Toward the solitude of Shaddar-Nur" then go east)
7) UW: Pirate Island cave (north of the portal island -- use the cave or outside the cave as a reverse cave
8) UW: Valley of Tears (VoT) Sakkara Nest

The cave (or reverse cave) gives the player a way of controlling (speeding up) the respawn of an area. If the area is thick with monsters, then fast exp. gain is possible.

Boss monsters give an exp. bonus (and have better loot possibilities)

VoT: Sakkara Demons give best exp. of all the monsters in the game.
Basically, the deeper you go in to the game, the more exp. the monsters provide.

 

Original Source: MP Exp. Guide by Manziel @ inSacred.de

My translation (FrostElfGuard) of Rotluch's translation, has probably introduced some errors, blame me, not Rotluch or Manziel.