It's called "massed attack", but it can be done only against Presidios and Bastions, with units from 1 or 2 other members of your own BH, and you need to have a sufficient BH rank (Swashbuckler or higher) to be able to send (for participation, when someone else is sending, you can be any rank). You can find more info in the in-game help system.
Yeah seems pretty dead in that area of the game doesn't it? I didn't even see a target when I looked today. I never really played with the suicide squad that much. First mistake I ever made as a new player was putting it all down on a LH target and watching a whale gulp it up. I I think it was "trotskinski or trotskentski." something similar to that. It was the yahoo era so hard to recall it clearly.
I've seen it done. Did it myself in the very early days, for raiding capacity, but these days it's a bit hard to understand why would anyone do it.
Some people have interesting strategies. For example, these units were sent earlier today to raid a player in my BH (I repeat: raid him, not defend against him!):
So yes, some people do send def - and even scouts - on raids. 😁
However, unless you are absolutely certain that you want to do it and you have a very good reason why to do that, I would suggest against it.