There are three Team Up Missions, also called Elite Coop Missions.
You can only play it with a partner, unless you use this cheat to play alone.
The courses are rather long, with two long waves of enemies, each ending with a miniboss.
One feature which will make these levels much easier, is that your partner can revive you once a game.
In two of the missions, Sky Strike and Deadly Grid, you will early in the mission encounter a rather heavily fortified kind of maze in the sky. Most brobots and overdrives can take down, or at least damage the grids to sneak through. To clear the obstacles entirely, you will need two overdrives or specials from brobots.
Each run, whether you invite to it yourself, or if you get an invitation will give you 40 bropoints.
Other than that, you will get the usual stuff like credits, some boxes, etc.
You will have to survive for quite a long time, so pack good armor, and armor regeneration. Also you will want good screen clearing abilities.
The minibosses are not the strongest, so rather go high on armor and power, than on boss killing abilities.
Also focus on planes and brobots with wide spread of bullets, rather than narrow streams, as the enemies come in wide waves. Homing missiles or bullets are very nice here.