Phone number or other contact info. Supposing you’re on good terms, they’re not someone you’re interested in romantically or the like (so none of the anxiety from that at least), what’s kept you from asking?
Supposing that something has on occasion, anyway.
If you’re having trouble building these networks, find a reason to have further contact. Follow up events, sharing a photographs, notification for the next event, follow-on event next weekend, have a discussion with somebody and say you’ll send them more information, many good reasons