pvp question

[SethWms]

when i enter pvp, (both times) I've seen an enemy force on the map, but also several green and teal forces on the map as well.

I attacked the teal force today, not out of choice but because my bolt throwers were in range.
I also noticed my opponent took control of one of the green forces (at least I think he did)

So, I guess I'm asking, are these forces all persuadable to join your side? Or just certain ones? Do you need a certain unit in order to do it? If I take over a stronghold on the map, can I then use that stronghold to hold out in (i.e. operate it's doors, build or place bolthrowers on towers, etc.)