Here's the short answer: No, only one person — the proposer — has the ability to edit a draft and propose it.
The long answer: This constraint is in place because it ensures that all participants have an equal opportunity to contribute to the proposal’s evolution as opposed to some participants having the opportunity to weigh in and suggest changes before others. The Murmur team is evaluating a form of collaborative editing (that doesn't devolve into chaos!) as a potential future feature. If you have a use case you'd like to share around this, we’d love to hear about it.
Remember, if someone else has proposed an agreement about something you're hoping to make better, you can propose changes to that agreement before it expires!
And another feature coming soon that will enable more co-creation: