That depends on the team and what duties you're trying to divy up, as well as where you are in the game. For example, in the early stages of a campaign everyone in my group tends to do their own thing after we agree which worlds to initially take. Later we start to assume more specialized roles, generally with one of us handling microing the fleet, another on defense, and the last person handling misc. details such as microing support ships and reinforcements.
As far as structuring your play into more specialized roles, that depends heavily on unlocks. For example, if a player intends to go ship heavy, we give him first pick of metal heavy worlds. Likewise, someone handling defense gets the crystal heavy worlds. If you end up going hybrid, you get the more balanced worlds. We also tend to shift forces between players, so the defense heavy player might give low level turrets or turrets unlocked by everyone to another player and carry on building more where needed. On the whole, delegation works pretty well as long as you have a team willing to work together.
That said, if you aren't willing to co-ordinate with other players and trust them to handle their responsibilities correctly, then you end up in a bit of a pickle. My roommate, for example, doesn't like other players microing his fleet, which makes co-ordinating offensives... interesting (though his computer has problems joining games, so it's pretty rare that he gets to join in).
