I took my 3 through Northrend, and did all the quests in each zone to get the achievement, and dinged 80 in Storm Peaks. You take a HUGE exp. ding in a group, solo you can ding a lot sooner than that, especially if you pug the instances when you can. With 5, you can run instances to make up for the lost exp, I'm doing that with the team in Outlands.

Speaking of my Outlands team, I took a break from them, to level up my rogue, who's my main miner/jewelcrafter, and running her with the 80 druid, it's 3-400 per kill grouped, 1100-1200 solo. (I just don't know how viable she'd be in the team, although her DPS would be a boon - but I want her making jewelry and gems later on.)

I'm thinking it's best to not group in Northrend if you have 2, and tag-kill, so the lower level toon gets more exp? It's working with my rogue/druid combo, because the rogue will stealth/pickpocket (gotta get those lockboxes!)/attack, then the druid drops into the fight with her Boom attacks. Works fine, and i'm getting a TON more exp. that way. It helps that I don't have to worry about quests on the druid, too, and she's a leatherworker, so she's skinning and outfitting the rogue as we go.

I'm still in Borean/Howling, will ungrouping the two lead to issues later on with phasing?