The scrying spell seems pretty specific in that it uses the phrase "You can observe a creature at any distance." but remember that the focus for this spell is either a mirror or a pool of water.
Can you allies gather around to watch what you are scrying? If so, could they cast spells like Message through the caster's Focus/Sensor? Would they gain access to the caster's senses such as Darkvision?
I'd like to know as well - from what I understand, the spell creates a new "sense" for the caster, which doesn't seem like it would go to allies. However, the focus of water/mirror makes it seem like the scrying is happening on the focus itself, which leads me to think that others could peer through it. I don't think they'd receive darkvision/low-light from the caster, since they'd be looking with their own eyes.
I think the answer is in the description. "You can observe a creature at any distance."
Just because it is focused on a mirror or water doesn't mean your allies can see what you see in the water. Perhaps they just see water. Or their reflection in the mirror.
I think the answer is in the description. "You can observe a creature at any distance."
I think I agree with this, although I like the idea of a Scrying spell that allows the caster's allies to also see what is happening through the focus. I think I will make a house rule where the spell can be cast at +1 level to allow for this (and also for the allies to share the caster's senses.)
While the rules don't explicitly support others seeing your scry results, I think it is a reasonable assumption given that you are 'focusing' your magic through the focus of the mirror/water.
If you are of the camp that says this is against the rules, a simple work around would be to cast Silent Image directly after casting scry to show your friends what you are seeing.
As far as others casting spells through your scry focus, I would probably say no to that.