I'm no expert - only being a mom for a couple years now, but I think there should be some basic unwritten rules of playdate etiquette. ALL playgroups are different, no one rule is going to fit all or even most of them. This is my personal input from not only running my fair share of playgroups but also being apart of too many to count:
Be dependable. Confirm before the date. Try to be on time. Let's face it, we're parents. Things tend to go a bit crazy when we're on a time frame. Make sure you have at least one cell number to text your tardiness to. If unable to make it - call the other parent(s) as soon as possible. Their time is valuable too.
Communicate. Host & Guest - establish time periods. It's amazing how important adding an end time to your home playdates is. Yes, you may love your friends conversation and your kids are getting along famously...but someone needs a nap or lunch soon so skidaddle.
Keep your cooties to yourself - Stay home or cancel if you're hosting if you or kiddo are sick! When in doubt, stay home. There's no reason to guess if its an allergy when you may be infecting other kids if your wrong. Don't bring an already over-tired toddler either.
Snacks should be potluck. All kids like different things and you can't expect the hostess to not only open her home to kids AND know what all the kids will eat. Bring something you know your kiddo will eat.
If attending & your child has an allergy - bring their own safe snack, its doubtful the group members will have all the kids allergies memorized much less remembered to tie their shoes on the way out of the house. It's the moms responsibility to make sure their child doesn't bee line for the peanut butter treats if they are allergic. Bring your own sippy cups.
Guests - Talk to your child about the rules of behavior before you attend. Don't micromanage but do follow through and monitor your child.
Guests - don't take it upon yourself invite other guests. Always confirm with the hostess additional guest such as kids who are out of school and family members who are in town.
Guests - Ask where your host prefers diaper changes to be done & about disposal.
Guests - offer to help clean up before you go. Make sure your child isn't slipping out with a toy that doesn't belong to him/her.