Unless your friends pull up in a tour bus, I don't see the harm in it. You will of course have to get towels fro them or they may bring their own. There's probably is a rule written about this but let's be honest, if I show up in my 2 bedroom unit that sleeps eight with three people, the accomodations, pool included are meant for 8. That's how I would rationalize inviting my friends for a day at the pool.