For collaborating remotely, I use a lot of email, shared google docs, private blogspot blogs where I can invite other authors and even Twitter. Of course, Slack, Trello, Github and other tools are popular in the development community. If possible, use something familiar. Trying to learn a new tool can be a project in its own right. If it isn't critical to the success of the project, don't do it. It just becomes a means to procrastinate.
If you have nothing, start with a design doc. If there are no roles assigned yet, take the initiative. Set up a file, start a Google group or a Slack channel, list goals and start hashing out who needs to do what.
You need a central repository. It does not have to be fancy. As your project grows, layers of organization can be added as the need for such becomes clear.
Just start. No more excuses. Expect the first attempt to be ugly, bad and stupid. Do it anyway.
Best.
OR just enjoy the vacation for a month and start when everyone is back, some people just cannot be trusted to step up and work unsupervised unfortunately...