Henchmen/summons/familiars/animal companions remove 20% of experience each. So 1 henchman is 80% XP, two is 64%, three is 51.2%, etc.
Players are split evenly. Two players get 50% experience each, three players get 33% experience each, four players get 25% experience each, and so on. Some modules/worlds adjust it but not in the original campaign.
That said, you should still be fine provided you do the Prologue first. You'll level more slowly (though quest XP doesn't change) but because you have a large party it'll make up for the level loss for each person (two level 8 characters > one level 10 character). The further you go the less of a problem it will be. Since you'll be lower level, the monsters will be worth more XP than they would be if you were higher level.
The game will also *try* to spawn more (and tougher) enemies for a larger group which is more experience, but that doesn't always succeed perfectly.
Bottom line: it'll still be easier than playing solo, but each of your levels will be lower.