So, let's have a look at the other mass combat BG.
Just as when comparing WSG with TP, you'll notice how Alterac Valley is positively huge when compared to IoC. Just as with TP this means you can actually change your mind as for wether going defensive or offensive.
As with Alterac there's very little you can do as well as any other role until the enemy boss is engaged. Go for a siege engine and get those gates down, or follow a parachut drop team to get inside and do the same job withou having to get gates down.
To a minor degree you can defend objectives, but the fight lasts for so short a time that taking enemy held objectives are rarely worth it.
You should expect the team rush madly for either the workshop or the docks initially. Preferably they should do both, but in a random BG with 40 players on each side there's very little coordination going on.
You'll just have to go with one group. Preferably the smaller one as you could make a difference in how long it takes your team to finally lose the initial struggle for that objective, and time is worth gold.
When the time comes to tank the boss, swap to PvE gear, pray enemy players are removed and make sure the boss stays in his room. You don't want the wiping buff to kick in. Use your raidwall (if specced into it) for the first jump (one of two boss special attacks) and drop your area heal at the same time. It's helpful if someone in the assaulting team could drop a bloodlust or similar just after the pull. Hence it's a good idea to communicate this on chat before pulling.