What is the difference between friendship and a friendly relationship between you and strangers?

What is often translated as „friendship“ is greek „philia“

Like in the word „philosophy“

It is a kind of love or attitude to others, so has a much broader meaning than having a few friends:

Philia - Wikipedia

en.wikipedia.org

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One thing that I do not understand though is whether, in Epicurus' time, there was such a thing as "social trust" among communities larger than say, the Garden, and whether this is thought to possibly contribute to a similar sense of security.

The Law of Xenia, or hospitality, was regarded as sacred to Zeus himself, and stories abound of the punishments inflicted on those who violated it. This unwritten law was the cornerstone of the Greek idea of social trust, and of the etiquette surrounding the host-guest relationship.

My favorite example of this kind of social trust is related by Vitruvius in his De Architectura:

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It is related of the Socratic philosopher Aristippus that, be­ing shipwrecked and cast ashore on the coast of the Rhodians, he observed geometrical figures drawn thereon, and cried out to his companions: "Let us be of good cheer, for I see the traces of man." With that he made for the city of Rhodes, and went straight to the gymnasium. There he fell to discussing philosophical subjects, and presents were bestowed upon him, so that he could not only fit himself out, but could also provide those who accompanied him with clothing and all other necessaries of life.

As soon as he saw the geometrical figures drawn in the sand, he knew he was near Greeks. He knew that he could trust that the Law of Xenia would be upheld, and that he and his shipmates would survive the ordeal.

Principle Doctrine 39 may be the most relevant text on this question:

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39. He who best knew how to meet fear of external foes made into one family all the creatures he could; and those he could not, he at any rate did not treat as aliens [ἀλλόφυλά]; and where he found even this impossible, he avoided all association, and, so far as was useful, kept them at a distance.

EDIT: I didn't see Joshua's post before posting this.

The question is an excellent one and I am still thinking about what to write further.

As we see in other discussions that focus on translations, there is considerable peril in looking solely to dictionaries and taking definitions out of context. Especially in the case of Epicurus, he uses words in non-standard ways when he believes that the popular opinion of it is inaccurate (example - "gods"). So you have to really be aware of the sweep of the remaining texts to make sure you don't take something out of context that contradicts the whole.

I would think the best way to proceed would be to first look at what we know of the actual lives of Epicureans and how they lived in the ancient world. And even after we do that, clearly there are lots of shades of intensity of friendship so that those so that those at one of the spectrum are hardly recognizable as being in the same category as the other end.

That's what you are illustrating in this question:

Quote from Noah Calderon

Are there bounds to the concept of friendship in the sense thought of by Epicurus (the characteristics of it and the specific benefits it provides) that rule out other forms of kinship with human beings, kinds that do not even necessarily require knowing the other person?

Surely the answer to that is "yes there are bounds" at least in the sense that there are differences in the way we deal with them. It was the Stoics that asserted that there is a divine order of things whereby all humans are bound together in divinely-ordered ways. Whatever else we might say about it, the Epicurean view seems to have been much more practical than that, and would turn on the specific people, places, times, and cirumstances, just as does "justice" (as explained in the PDs).