koinōneō

This verb comes from the ordinary Greek word for whatever is "common" or "shared" rather than private. It does not describe warm feelings or pleasant company. It describes an actual joint holding: when the action is done, two parties have a real stake in the same thing.

Because what is held is held jointly, the word works from either end, and English has to choose a direction for it:

• One party takes a part in something that already belongs to another, as the children have flesh and blood and as believers have Christ's sufferings.
• One party hands another a part of what is his own, as the Philippian church supplied Paul and as those taught supported the one teaching them.
• One party is drawn into someone else's wrongdoing and comes away with a stake in it, which is why two passages use the word as a warning.

That last use is worth pausing over, because it shows the word is not automatically a happy one. Sharing makes a bond, and a bond with the wrong thing implicates you. To greet the false teacher, John says, is to have a share in his evil deeds, and to lay hands on a man too quickly is to take on a share of another man's sins.

The noun built on this verb, κοινωνία, names the sharing itself, the partnership that results; the verb names the doing of it. A nearby verb, μεταλαμβάνω, sounds similar but usually runs one way only: it is receiving a portion, most often food set in front of you, with nobody sharing it from the other side.