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.