@Emily1767
I'm kind of repeating what others have said, but it really does vary from host to host how they feel about this. Some hosts have a policy of strictly no persons on the property other than those on the booking (for insurance, and other, reasons as has been explained), whilst others actually set up and advertise their listings as being good for entertaining friends.
Firstly, make sure you have read the full listing and house rules as you might already find your answer there. If not, simply ask the host if it would be okay. Unless the listing explicitly states that this is okay, I would always ask the host out of courtesy anyway.
Besides not being unexpectedly charged extra, you don't want to upset the host and get a bad review. Many future hosts would be put off to hear you had unauthorised visitors, especially if it happened on a regular basis.