In my discipline in the social sciences, book chapters are typical outcomes of workshops or of (repeated) conference panels. The book editors usually organize the workshop, which includes funding, establishing contacts and sending out invitations or an open call for papers. As a participant (rather than organizer), it is difficult to know from the outset whether the workshop will result in any form of collective publication, but sometimes the intention to work on a publication is specifically advertised. As an organizer, this is in your hands; and it of course leads to several follow-up questions (how to edit a book etc.) that are beyond the scope of this answer.