Have a seat, guys
I think "have a seat" is a fixed phrase that does not change whether you address someone or a group of people.
Is that correct?
If you have more to add to the post, please do so as I would love to know more.
It is correct, but there's more to it than just that one phrase. Any other action phrase can be used in the same manner.
When telling a group of people to perform an action, it is not required to refer to the action or the objects involved with it in the plural. In fact, it is more common to leave the action phrase singular, than it is to make it plural.
For example, you would more likely say...
Everyone raise your right hand.
...instead of...
Everyone raise your right hands.
...even though both are correct.
Now, if the objects that the action uses are plural, then you must refer to them in the plural, for example when you want everyone to raise both of their hands instead of just one:
Everyone raise your hands.