As indicated in a previous post, we have gotten inconsistent and conflicting information about the answer to this question from both Verizon CS reps as well as in-store personnel. Everything from "shared is truly shared and it's the accumulated detail total for all participants" to "each participants data usage is rounded up so with three participants, you'll need 3gb minimum, assuming each participant uses any data at all".
Visited a store (again) today and got a rep who was absolutely positive that there is no individual rounding up and if user #1 uses 75mb and user #2 uses 100mb and user #3 uses 225mb, the total data usage is 400mb and we would be under a 1gb shared plan usage. He volunteered to double check this with his manager, which he did, and he assured us his information was accurate and correct. He seemed to be the most knowledgeable and forthright rep we've spoken to thus far.
We have three smartphones and in tracking our monthly usage we are confident we use very little data and that a 1gb plan will suffice for our needs. Much of our usage is wi-fi (versus Verizon network) and we are careful to use wi-fi whenever available.
We're going to switch plans. If it turns out he lied (or was misinformed) I'm willing to pay the contract buyout price to dump Verizon and switch to my daughter's Sprint family plan for $30 each, with unlimited data and talk and text.