I don't know if you can join a friends game or not. I think you have to be invited, not sure. Every time I checked the 360 Brink tag last night, some peoples matches said joinable, but I had no option to join the match. If I am online and someone needs a medic, send me an invite Serpico MTZ.
I played quite a few lagless 8 vs 8 matches on 360 last night though and they were hella fun. Just searching for a standard, objective match. That seems to always throw me into a full room. The game plays great without lag though and really shines with full human teams. One of the greatest moments was me capturing an enemy command post only to step on a mine that someone had placed there. I stood still and in about 30 seconds a friendly engineer came over and disarmed the mine I was standing on. I had to send him a message thanking him for it, it was great.
Slide tackling groups of people from behind is awesome. I am also getting my grenade cooking skills down, they are really useful when used right. Mainly sticking with a light bodied medic so far, I love the health buff I can give myself and the fast movement of the light, is a must for me. I am constantly running around reviving, sliding into cover, non-stop slide tackling fools, really fun.
One thing more people need to do, is watch your corners and use the lean function. No need to expose your full body going around/checking a corner when you can just do a quick peek around the side of it. Really like that they kept the lean feature in for consoles.