With the way things work, no one person can focus on defense and that's that. EVERYONE has to defend, and preferably all on one planet which has the only open warp link. You can't afford to not use all your team's resources to defend. If everyone is defending separately, you're making it far far far FAR harder than it could be.
Everyone on the team should have the majority of their turrets/fortresses/other defense on one planet for wave defense in multiplayer so none of their effectiveness gets wasted. At the very least pile up everyone's lightning and tractor turret mk 1s
Offense is a little more forgiving, but you should all really all attack at the same time all together with a full ship cap each time post by post.
Edit: Also the missile turrets, snipers and standard turrets. Frigates are immune to AE.
