So take this one with a huge grain of salt, as it's one of those he said/she said kind of things. According to a post on CoG there was a link on a Norwegian board to a Gamespot article detailing a bunch of features for the upcoming Mass Effect sequel. Amongst all the nugget of goodness that will be in ME2 is a new expanded squad (up to 5) and the ability to bring human players into that squad for full 5 player co-op.
Some other rumored features of interest include the ability to import your Mass Effect 1 character, a raised level cap of 120, and weapon customization. According to the post the game is set to have a 6 month timed exclusivity deal with Microsoft for the Xbox 360, because even though Microsoft Game Studio published the first title, Electronic Arts now owns the rights.