There is some flexibility here, depending mainly on the volume of submissions. If in doubt, give it a try. The list of groups below includes many who are at extreme odds with one another. If you are organizing an event and for some reason don't want news of it to reach those of unlike minds, best not to submit it to WalkieTalkie. In addition to the calendar of events, WalkieTalkie will include a section for groups to coordinate efforts for envisioned activities.
Groups can switch contact people at any time in the same way.
The burden of this on contact people should be light. They may send information as often or as rarely as their needs dictate. The more advance notice the better. There is no rigid format for submissions, so forwarding email announcements used for other purposes will typically suffice. If your event isn't free of charge, it would be crummy not to reveal that.