I think, and am not 100% sure. But if you are the lower guy you'll be going against + whatever ships. So if your buddy is T4 you might see +15 ships. So for you that would mean a bunch of quick deaths.
I was grouping with someone five levels higher who fought NPCs that were +1 to him so I saw +6 opponents. I died a lot (and so did he) but that was mainly due to the mission setup: you spawn in a group of romulan BS and Escorts that immediately open fire on you.
I did get less kill XP than what I get during solo missions (when I solo kill a +0 BS I saw 10-14 points, the +6's during that mission gave 4-5), but that was likely due to the group and the fact that I wasn't normally eligible for the mission - the mission text mentioned reduced XP.
As for the difficulty: I later grouped with a fleetmate one level higher and we encountered NPCs that were +5 to me yet easily dispatched them. I don't remember how many points I got for them though.
If I, at level 45, team up with a guy at level 6 to do a level 45 mission, i will get the full 565 SP for that mission, but the level 6 guy will get penalized for doing a mission above his level and will only get 29 SP or so. Bottomline is, you always get the same range of SP that is avaible at your level, and never more than the episodes that are avaible to you.
If you are high level and trying to help a fleet m8 or something like that let them go ito a system first and spawn the groups etc then you enter and help them they will fight at the same lvl as they are.
I myself have helped some fleet members to do this.
I've been doing this with a friend, I was Captain 2 and he was 11 levels lower than me. I was killing ships and ground mobs that were also 11 levels lower than me. Although where there should be only 3 mobs, 6 would spawn instead. Made no deferents to me as they still go pop just as fast.
I've been doing some missions, exploration and such with friends, and sometimes ships will spawn at the level of the highest level player. So be cautious if a higher level friend is helping you and check the level of enemies before you rush in.
I'm currently Commander 6 and went a few days ago to help out my lower leveled (Lt. Commander 3-4) friends. This is what I encountered:
-Story missions seemed to have set levels. We were constantly fighting -15 or so mobs to me. The spawns were doubled by there being two players, but everything was still at mission level. Very easy for me to blow everything up.
-Exploration missions seemed to spawn at the level of whoever zoned in first. We did one of my explorations and got decimated (double-spawns at +/- 0 to me, +12-15 to them). The next mission was the lower level's and the mobs were at his level if I waited quite a bit before zoning in.
Overall, it seemed best to just have them solo any exploration they wanted to do and I could help with storyline missions. The added fact that exploration missions are a minor pain to get shared *anyways* doesn't help.
I've found story missions always spawn at the same level, but will scale up based on how many people are in the team. The exploration missions, though, will scale up to the highest level player's level. Can be pretty painful.