The ITICSE conference of ACM has topical working groups that meet alongside the main conference. This has traditionally been face to face interactions, but is being adapted. But they also have a mailing list that goes out to a wide audience to announce the conference and to seek participation - call for participation. I would guess that most conferences have such a call. The mailing list consists of anyone who has expressed any interest in the conference in the past, including presenters and attendees. Sometimes it goes out to everyone in a special interest group such as SIGCSE in the case of ITICSE.
Get you call for working groups mentioned in the call for participation and have someone field all suggestions that come in and send back out a list of those chosen for inclusion and how to join. In a face to face conference the rooms available may be limited so only a few working groups are accepted. You need to decide whether some limitations are appropriate or not and, if so, how to choose the once that will be included.
Each such group will need one or two "conveners" who are responsible for accepting participants, perhaps, and also for keeping things on topic. They might also be responsible for producing a summary at the end of the conference so that any results can be shared, perhaps on the conference web site.
If the structure is elaborate enough the call might need to come in two parts. First a call for topics, mentioning who will convene each one. Then, after topic selection, a further call for participants. More emails and/or more web site updates.
The call is also normally put on the conference web site if any. But most people hear about it via email, I suspect.
Talk to the conference chair about getting included in the call and on the web site. If necessary talk to the leaders of the society that sponsors the conference if any.