From my own experience I'd say that unpaid online teams only really work if they consist of not more than three persons, as it keeps the communication simple and it will always be clear who did what.
The alternative is paying the people working with you, which won't give you any real guarantees either, but is an additional motivation for everyone to actually work on your project.

This is really interesting, because my team totaled six people in the end, but I considered our "core" team to consist only of three people. These three people (which included myself) easily contributed the vast majority of the work done on the project in the end. Also, I was always really wary about bringing on new hands for the reasons you mentioned. Going back to the point I made in my last post, I do not recommend bringing on anyone until you are sure you need them for the project... most projects do not need more than one person working on them until construction has begun in earnest anyway.