When one states "all the participants", one needs to distinguish between all who contributed, or all who are currently contributing. "Active signatories", like Falconius said. It won't be a month after the first map WIP is posted, before somebody not only drops out, but does so in an untraceable, unreachable manner. Maybe it would be sufficient to say if you want to retain a vote in any decision, you have to stay in touch. Decisions may need to be made fairly quickly... people don't have to be vanished to be out of a given decision - most of us go on trips, study for exams, spend time sick, or a thousand other reasons for being unable to communicate. Perhaps the project needs to state up front that no intellectual-property decisions will be finalized in less than X number of days - that way folks can be sure to check in every X-1 days to be sure they never miss out - if it matters to them.

I've been part of cooperative worldbuilding where people who withdraw try to take their stuff with them... that HAS to be understood to be impossible from the start. Not in a use-it-elsewhere sense, but in a remove-it-from-use-here sense. An up-front agreement that in whatever way is concluded in this thread or successor discussions, ALL material contributed remains useable by the project as a whole, in perpetuity. I suppose the retention of A license to use your own stuff might be fine, just not THE SOLE license to do so.

Perhaps a coordinator role needs to be entrusted with more than the usual single in-Guild means of contact; again, if someone wants to be solidly in touch. If copyright and use issues don't much matter to an individual, then the board or private messages may be enough.