Here in Denmark, I think it is kind of a mix... my general impression is also that it is a mix in my neighbouring countries and such.
That friends do get invited to big events, weddings, deaths.. maybe an occational birthday.. however, no, they don't normally come to family gatherings, that is almost a "no-no", I remember once when someone whom even was somewhat family uninvited and unexpectedly joined a family get-together, and we didn't like much, his personality however maybe also had something to do with it, and he ate a lot of our food without bringing anything.
A friend can often come along to whatever, but it is not the norm, no.
The smaller the group in a way, the more unlikely a friend will be a long.. if i had a girlfriend and we went on a trip, then we wouldn't have a 3rd wheel friend, because it's like private and special to us. Maybe if we were to just give a ride half-way and dump him or her before our destination, sure.
When we go on ski-trip with lots of family it's more normal that someone bring along a friend. That is also more about some activity.
Naturally when younger, then friends come along a lot.. and maybe when older it is not so much a problem, tho not the norm.
....
How I treat my friends?, I hope good.
They are also my source for support when things get rough in family.
Actually, i thought that in middle east, and areas around, that since it is more tight, that things are also more private, that you do not involve anyone from outside. That it stays in family and or select friends. Which also makes sense enough at times, and good.. but not always, for example when taking law into own hands.
But it is also commonly know that Europeans can be somewhat closed off.. if someone rides a bike, or walks thru europe, he wont experience much that people invite him or her into the house for a meal or bed.
Op's but that didn't have so much to do with friends.