You should enjoy your stay abroad as much as possible, so we hope your family and friends will come to visit you.
If those family and friends can sleepover at your place is really up to your host, Student Room Flat has no power to make any decision regarding guests. We highly recommend you to discuss this possibility with your host during your check-in, or as soon as possible during your stay.
Most of the time for insurance, hygiene and security reasons hosts do no let tenants have sleepover guests. The flats are made for the number of flatmates (size of common spaces, number of toilets, bathrooms, etc.) and cannot accommodate other inhabitants.