The same way you form any group: you meet regularly. Set a time and place that seems like it would be good for a good percentage of the people, and turn up there, every meeting. Have some of the meetings be about a concept or theme; let everyone know what the concept is ahead of time, so they can think about it and maybe do research or bring examples. And have some of the meetings be open meetings, where anyone can talk about stuff relevant to the group's purpose. If it becomes a more active group, you may need to limit talking time per person in the open meetings; and if a particular topic catches fire in an open meeting, you can revisit the topic in a themed meeting.
But groups are formed the same way friendships are formed: people turn up regularly to spend time together. Some meetings you may be the only person there, but be open and welcoming to those that do show up.