You could run your start up as a co-op.

Speaking from experience, Loomio is a wonderfully effective decision making platform. :)

how does it ensure that only one person gets one account?

We're interested in exploring cryptographic anonymization. But the truth is, most groups don't need it.

What about using SMS/Facebook auth?

for auth at the moment we have (in no particular order): google, facebook, persona

groups can set their own threshold for entry. one option is manual approval where the applicants email is visible to coordinators and the applicant supplies an introduction. simple but quite effective

it doesn't. The use-case is high trust groups not mass voting.