We don't know where the other headquarters personnel went, but they are clearly not with Leia, Han, and C-3PO when their way is blocked by a cave-in. Maybe they went a different way to get to the transport, or maybe they had gotten ahead of the main characters before the cave-in.
In any case, it is not not Solo's responsibility to keep track of where they are. The captain and crew of the transport seem to be waiting on Leia's arrival, and so Han informs them that she is not going to be able to make it to their takeoff site. Presumably, they are waiting for the other headquarters staff who are assigned to that vessel as well. Han telling the transport to "take off" does not really mean that it is going to leave immediately, stranding people still left on the ground; it just means not to wait for Princes Leia.
Finally, we also know that the last transport is not the last vessel to leave; there are other, smaller craft still on the ground at that point. The Millennium Falcon is one of them, as are the small fighters. While there isn't enough room to catch a lift in an X-wing, there may be other intermediate-sized vessels capable of taking on additional passengers if they do miss the last large transport.