We dont have to many here but I am not all for them. (Dog Parks). I think they are a breeding ground for stuff, place for the unexpected to happen and many more....
For our playgroups, I look for people who have boxers. I meet the dog and test the temperments and then find a place to have a playdate. No dog in my group is there that I have not met before or know their lineage or where they came from. Precaution because I am looking out for my dogs.
I tend to only play with those I know. I know may sound snotty, but in a dog park there are dogs there that you do not know their temperments, nor their owners temperments let alone history. Best IMO to avoid these all together.
I try to have monlth playdates or get together with other boxer owners in a smaller environment.