From the most recent Statesman interview (formatting and [annotations] by me), some good basic info on experience and teams and SKs and Exemplars:
Question: Could you give a complete overview of how experience is awarded when in a team? How does the level of the team members affect the xp earned and how it is split between players? What effect does Sidekicking or Exemplaring have? Is there a group bonus, and does it change based on the size of the group? (Apparently much of this has been explained in dev posts on the US boards, but as these are no longer available, new European players like me would really appreciate the info.)
Answer:
On the top level, XP is divided by damage done. If two heroes both attack and defeat a single mob, each hero is given XP proportionate to the amount of damage received by the mob.
Within a team-up, XP is divided equally among each member, but levels are taken into account. So if a 35th level Blaster teams up with a 32nd level Tanker, the 35th level Blaster would receive proportionately more XP (on the assumption that the higher level character contributed more to the conflict).
When two or more different groups of heroes attack the same mob, the damage of each group is added up and used to divide the mob’s XP.
For every person in a team up, there is an XP boost. The experience points of each mob are multiplied by a value before being divided among the team members. Currently, the XP boost values are this:
Team Size – XP multiplier
2 – x1.25
3 – x1.5
4 – x1.8
5 – x2
6 – x2.1
7 – x2.2
8 – x2.5
[So team-ups divide the experience from a mob, though there’s a slight compensating multiplier shown above and certain tiers of team size will increase the overall numbers of mobs, and, thus, aggregate XP. Still, the truism that the smaller the group the better the XP, dying-horribly-and-taking-debt aside, is true.]
Sidekicking allows players to play at a level higher than their actual Security Level. While Sidekicked, a hero is considered to be at this higher level for XP division. But the hero receives XP depending upon the relatively levels of the mobs he faces. Example: my 15th level Scrapper is sidekicked to a 28th level Controller. For purposes of combat, my Scrapper is now 27th level. If I went out and fought 29th level minions, their XP would be calculated as if they were 17th level minions.
[Thus, the SK doesn’t get XP as though defeating folks 14 levels higher, just 2 levels higher.]
One thing to keep in mind is that as a sidekick, a hero is counted as a full high-level hero when doling out rewards even though he doesn’t have as many powers as a natural hero would. So the risk for the mentor (and teammates) is slightly higher but they get the same reward. If you are within 3-5 levels of your buddy you might want to think about being a regular teammate and not a sidekick: the rewards might be better. This will depend on the villains you are fighting, your aversion to risk, and the composition of your team.
While Exemplaring, a hero receives XP in a similar way. The hero’s actual level is brought down to that of the Aspirant. The Exemplar takes XP from mobs depending the relation of the Exemplar level and the mob level. Example: A 45th level Defender is Exemplarred down to 29th level. He fights a 30th level Minion. The Defender earns XP as though it were a 46th level minion. However, an Exemplar does not gain XP; anything earned is applied exclusively to XP debt.
(via CoH LJ)