We always have an open door if people want to pop in. No need to ask. Just come on in. This is the same for family, our friends and DS's friends.
Our basement has become the main spot inside that DS and his friends hang out in.
I was out today and the woman I was with said that this was really strange and no way she would allow her kids (when older) to do that. If they wanted friends around they have to ask before. And even then numbers would be limited.
What is your 'policy'?