The fields pertaining to making a new account—like claiming your makes.org URL—are synced across everyone in the session. So if two new people come in to edit a document, then try to create accounts at the same time, chaos will ensue.
There is, right now, no way to tell TogetherJS to ignore certain elements, however there is a (slowly going stale) PR for TogetherJS that adds that in, although it needs a cleanup (https://github.com/mozilla/togetherjs/pull/879). It may not be a bad idea to help clean that up and the use the new functionality that gives us to solve this issue.
Closing as part of maintenance: thimble is getting a full makeover, and issues will be tracked on http://github.com/mozilla/thimble