A friend did two workshops last year.
In both, few participants knew each other.
In the first workshop, she divided participants into groups of five for the day. She would explain a concept and they would work through an exercise together. It went well. She noticed the participants generally had lunch together in their workshop groups. A few weeks later, a few were still in touch.
In the second workshop, she tried our suggestion. She again divided participants into groups of five, but mixed up the groups for every exercise. The goal wasn't just to increase diversity of opinion, but to increase the first-contact familiarity.
Once you've spoken to someone, just once, you feel comfortable speaking to them in the future. If you enter a room and see one of these people, it's not intimidating to say hi and catch up. It's easier throughout the day and any future events to initiate a discussion.
The challenge is to have that first-contact familiarity.
For all the talk of humans are social creatures, we have a big fear of social situations with strangers. For many people, it's scary to approach a stranger and introduce yourself. We need an excuse to do it. We need to be forced into it.
The former workshop allows for deeper relationships, the latter provides a platform for plenty more.
When she followed up two month's later, she found a far higher number were still in touch with one another, often bumped into each other at events, and planned to attend the next event.
If you're running an in-person event, plan activities that will force people to interact with one another. Make it impossible not to interact with lots of people. When people arrive, guide them towards different groups. Schedule an activity. People may be nervous at first, but they will thank you for it later.