A Imperial Trooper and a Battle Brother aren't going to be involved with a mission briefing, most likely. I'd assume the relevant officers, and your representative from the Deathwatch would be the ones getting the orders, to send back to their units, unless I'm missing something here?
Ok, I found the time to post. Next post will probably be reaching the landing zone, unless @Maensith throws something in my way. I also took the weirdest name, I could find in the generator, again
Probably atleast a squads worth. We will have a space marine after all, plus you can't forget the techpriest.
That's on the conservative side. More than likely he would have a platoons worth at the least scouring other sections of the station that his personal boarding party was not in.