So, despite playing Demon's Souls for 50 hours and being 25 hours into this game, I've never once teamed up with another guy. I've been invaded, but I played most of Demon's in soul form and I've been playing all of this game in undead form.
I get to the top of Sen's Fortress, I happen to have gained a humanity. I go to a bonfire to save and think the what the hell and use it since I thought I would die on the boss anyway and might as well use it before I lose it.
Then, outside his door, I see a summon sign. I use it, and a guy comes into my game. We go on to the boss, I run up to him and slash away with my Lightning Spear as he throws Heavy Soul Arrows at him. When he's out of magic he runs up and slashes him and I start throwing my Soul Arrows at him. He goes down super easily.
It was fucking cool.
I'm a bit undecided about whether or not to do this again, now. On one hand it really is awesome you can team up like that, but on the other hand there is a great satisfaction in beating a boss by yourself - everything I've done in Demon's and this game I've done by myself. That's a cool feeling.
I think I'm going to try to tackle other bosses by myself, but if I have a humanity handy and I might use it from time to time. Perhaps on bosses that really kick my ass.
Anyway, I know you've all done this a million times, but it's new for me, so yeah.