Our nearly 8 year-olds are free to be outside completely unsupervised. They know where I am if someone's bleeding, but other than that, they're solo. They also can ride their bikes up and down the block. (Should caveat by saying we live in a very safe neighborhood, with sidewalks on both sides of the street, and know our neighbors well.) They walk to friends houses w/in around a 2-block radius. They are not allowed INTO neighbors' houses without first asking me, and in fact there are steep penalties for doing this. Even our lovely elderly neighbor next door, if they go into her house to see her cat or whatever without first checking in with me, they lose a significant amount of freedom for a time and we talk all about it. Haven't done that in a while actually.
Our 5 year old is allowed in the backyard only unsupervised (again, by unsupervised, I mean, he's outside cooking around the sandbox or digging for worms, and I'm in the kitchen. He knows where I am and he is definitely in earshot but I can't SEE him 100% of the time (though obviously I check in on him regularly). He is not allowed out of our yard or in our front yard without first asking me. Again, the kids have had enough beautiful days spent inside (without TV) with mommy who has to be studying or has work to do to know how much it sucks when you don't respect the household rules for safety.
Rarely do they make a mistake on that anymore and like I said it's been a long time for the older two.
As long as they ask me first, then yes they are allowed to go to someone's house and ask them to play, or go inside for a snack, or visit the cat or whatever. I don't like controlling too much the whole asking a friend to play thing. If I thought it was a problem (my kid were at their house twice a day asking to play) I'd put some rules in there, but if it ain't broke, don't fix it.
The big thing we're working on now with the 8 year-olds is them recognizing when a previously allowable situation has changed. For example... the nice old lady is now in her backyard gardening and they're inside with the cat... the friend's mom runs out to the store leaving 2 8 year-olds in the care of a 16 year-old sister, etc. Basically I tell them, when I give you the okay, it's for this and this specifically... not a carte blanche to be there the whole day no matter what happens. I think this will become really important in the teenage years a la when the party goes south, or when the bad kids show up at the party, etc. then what.