We have a pool and my children have been swimming for about 7 years now. My youngest is 11. They always looked forward to having friends over and thus far we only had one incident where a child lied about their swimming abilities. Since the kid was old enough to be truthful about water I took their word. Long story short...I ended up giving swimming lessons that evening.
Now, unless they've been here before and already shown themselves as capable swimmers; THEY MUST BE ABLE TO SWIM THE FULL LENGTH OF OUR POOL AND BACK.
Our other pool rules which are ingrained in their skulls is:
1. Two people must be outside or inside the pool swimming at all times. NO LONE SWIMMERS
2. You must be able to hold your breath under water for atleast 30 seconds.
3. You must have floaties or a life jacket if you can't swim.
4. Absolutely no running around the pool deck.
5. No eating or drinking near the pool.
6. Everyone needs to bring their own towels and dry clothes. You'll be surprised at how many of the teenage boys will show up in just their trunks.
* TIP- if you live in an area that is susceptible to wind even when it's a million degrees outside then swim caps will go a long ways. Also, for the young ones as they tend to get cold quicker because the water can be a bit cold. It can be 120 degrees outside, but if that water is 70 or colder; that water will be cold and they won't stay in long.
Having said all that, the kids have a wonderful time. When we BBQ the parents don't have a problem keeping an eye on the younger swimmers if someone needs to run in the house. We've had more than 10 kids swimming at one time that were all decent swimmers (but, that's regulated by your comfort level). Everyone can't wait for swimming season to start at the end of Mar/early April.
It will be some great memories at any age. They really love to invite friends over and feel free. I don't mean to impose, but swim safety is a must.
Enjoy. Hope this helps.