The quorum is usually based on the documents that define the organization. The Constitution, for instance, defines a quorum for Congress, City Charters usually define the quorum for city councils and equivalents, incorporation papers set out the quorum for a corporate or non-profit’s board of directors, etc. Whoever is involved in the drafting and approval of those documents could be considered the ones that decide what quorum is. In the case of Congress, of course, it was the Constitutional Convention. In the case of a startup company, it could be the founders.
In the condo & homeowner’s associations I have belonged to a quorum is defined by the by-laws and is legally binding, I think (IANAL).
There is sort of a bootstrapping process here because somebody must first write the by-laws, so what is a quorum of that group that writes the by-laws? You got me there. I’ve never been around at startup time.
A mass meeting, or a newly forming organization that hasn’t yet adopted rules, is operating under the “general parliamentary law.” From Black’s again:
Many organizations operate without rules: perhaps the most common example is a legislative body that is convening for the first time after an election, and is thus not bound by the last session’s rules:
National Conference of State Legislatures, Mason’s Manual of Legislative Procedure § 39 at 34–36 (2000).
Likewise,
Henry M. Robert, Robert’s Rules of Order Newly Revised § 1 at 3 (10th ed. 2000).
According to every parliamentary manual still in print in the United States, a quorum under the general parliamentary law is a simple majority of all the members.