I am looking for the closest single word which refers only to those people who are directly connected to a person's life. Most specifically, this the meaning of the word should include co-workers, classmates, friend, and family members, but should not include strangers.
How about kith? I had always assumed that kith excluded family members (because of the phrase "kith and kin") but it seems to have about the definition and connotation you desire.
Definition of KITH: familiar friends, neighbors, or relatives
Of course the word isn't commonly used outside of that one particular phrase, but still, it seems to fit.
Here's the etymology of it.
Community is broad enough to encompass co-workers and friends, and intimate enough to include family.
community (noun) 1. a social group of any size whose members reside in a specific locality, share government, and often have a common cultural and historical heritage.
2.a social, religious, occupational, or other group sharing common characteristics or interests and perceived or perceiving itself as distinct in some respect from the larger society within which it exists.
In Australian English there is an evolving usage of the term mob.
Although the standard definition is clearly pejorative, the word has a history of affectionate (or at least jocular) use in Australia. When it was time for the neighbourhood children to return to their own homes, my father would say "Can't you mob hear your mothers calling you for tea?"
It would certainly be applied to family as in "When all the aunts and uncles gathered, we were quite a mob to feed. Talk about the loaves and fishes".
In recent years, Indigenous Australians have begun to claim the term as a collective noun for family, kinship clan, people living in the district or all Aboriginal people as needed. The website OurMob.com.au describes itself as "a premier Aboriginal and Torres Strait Islander employment and community information website" with the tagline If it is not on OurMob.com.au it is not reaching Our Mob.
I might try associates (which may sound a little stodgy for family members, but the word seems fairly flexible, according to NOAD):
associate (n.) 1 a partner or colleague in business or at work : he arranged for a close associate to take control of the institute. • a companion or friend : his old friend and hearty associate.
I think it could be applied to co-workers, classmates, and friends easily enough.
In its narrow sense, the word 'acquaintance' can be used to mean someone with whom you are acquainted and no more than that. For example, if someone asks if a person is your friend, you might say "No, more of an acquaintance really." Thereby, one could argue on that basis that acquaintances would not include friends. But friends are a type of acquaintance, just as a father is a type of relative.
If one accepts that the word 'acquaintance' is merely an antonym of the word 'stranger', because it is used to describe those with whom you are acquainted, as opposed to those with whom you are not, then in its broad sense 'acquaintances' is probably the answer you're looking for!
We hear so often about social networks. Since you want a single word, how about network. It has many definitions. The "social" definition is somewhat cynical:
As a noun: "A directory of people maintained for their advancement."
As a verb: "To interact socially for the purpose of getting connections or personal advancement."
But try it on for size. I think with "social networks" being a ubiquitous and au courant phrase, the simpler "network" might work for your purposes.
Kinship could also be applicable. While this normally denotes family and close relationships it could also be applied in a broader context.
Kinship is a term with various meanings depending upon the context. This article reflects the long-standing use of the term in anthropology, which is usually considered to refer to the web of social relationships that form an important part of the lives of most humans in most societies, although its exact meanings even within this discipline are often debated
A couple of suggestions that I don't see among the answers yet:
noun - definition: a small, intimate, and often influential group of people. Origin: 1870-75
noun - definition: group, assortment
array, assemblage, band, batch, body, bunch, bundle, camp, circle, clan, class, clique, clump, cluster, clutch, collection, company, compendium, coterie, crew, crowd, faction, gaggle, gang, kit, lot, mob, organization, outfit, pack, push, rat pack, sect, series
As you can see from the list of synonyms for "set", there are many, many words for a group that could be used for your purposes but each has another, more specific connotation that may not be appropriate.
I think the biggest hurdle in your challenge is to find a single word to tie together one's close circle of family, friends and the group consisting of co-workers or school mates which my kin in Germany so suitably call (I've translated) colleagues or acquaintances.
Familiars? I was thinking number 2 below...
Definition of FAMILIAR
1: a member of the household of a high official
2: one who is often seen and well known; especially : an intimate associate : companion
3: a spirit often embodied in an animal and held to attend and serve or guard a person
4 a : one who is well acquainted with something
4 b : one who frequents a place
Not sure if this is even close.
noun 1 [ mass noun ] friendly association, especially with people who share one's interests: they valued fun and good fellowship as the cement of the community.
• [ count noun ] a group of people meeting to pursue a shared interest or aim.
• [ count noun ] a guild or corporation.
Source: The OED
The correct answer is "Sphere" referring to a persons sphere of influence which is the most applicable word I know of that specifically includes different groups of people that influence your life and vice vera in a multitude of ways as well as different time periods or stages of ones life, all of which you have had some memorable personal interaction with.