The common denominator is to have shared spaces where it's expected to be among strangers' presence, and for those strangers to eventually become repeat guests in a person's life. That's the maximally comfortable scenario for inducing social behavior and it's responsible for eons of human social history. Think church.

The problem there is that it's the responsibility of groups or society to arrange that. There's not much that a single lonely person can do there.

The less common denominator, that an individual may partake in until society concocts a better solution, is to intentionally visit existing shared spaces even where they otherwise wouldn't (hint: bouldering gyms are good for this because there are repeat faces as well as a social okay-ness to congratulating strangers, or asking how certain challenges can be solved).

Or break with convention, comfort, and perhaps etiquette, and instead just talk to people. Even outside of those spaces. (This is the advice that will piss a lot of people off if it's presented as their only option.) This advice is horrible until it isn't. It does, with enough practice, 'just work'.

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For an entrepreneur or organizer: it would just go a long way to think about things in terms of allowing conversation to happen unimpeded. Pay attention to where people talk, and about what. Conversations happen a lot in hallways but famously by water coolers, perhaps because it affords people enough time in a shared space to muster the internal capital to start a conversation.

In college I ran a forum for people to meet others and some of the most self-reportedly successful participants just asked questions into the void and were surprised by the number of responses.