According to Robert's Rules, a quorum is the minimum number of voting members who must be present at a properly called meeting in order to conduct business in the name of the group. A quorum should consist of "as large as can be depended upon for being present at all meetings when the weather is not exceptionally bad." In other words, at best, a quorum is just an educated guess.
Establishing a quorum
Some groups set a quorum as a percentage of membership, and others use a fixed number. To determine what's best for your organization, you really need a track record for your group so that you can come up with a number that doesn't allow too few people to spend all the money in the treasury, or doesn't call for some number that's way too high. Unfortunately, most of the time you don't have a track record to go off of when you're just getting started. So, proceed with caution, and think about your quorum carefully. Use common sense and be willing to change the quorum frequently as your organization grows.
Until you make a different decision for your group and include it in your bylaws, Robert's Rules sets your quorum at a majority of the members.
Dealing with the absence of a quorum
No matter what the reason, sometimes too few members show up for a meeting. When this happens, you've got options, but they're pretty limited. That's a good thing, though. You don't want three of your members voting to divide the treasury between them and dissolve the association!
All is not lost if you're quorumless, however. Robert's Rules lays out four things you can do during a meeting in which a quorum is not present. You can
Fix the time to which to adjourn. Doing so makes it possible for the meeting to continue on a later day, after you've chased down enough people to achieve a quorum.
Adjourn. You can call it quits for the time being and wait for the next regular meeting.
Recess. Sometimes achieving a quorum is as simple as taking a short break to go out into the hall and round up more members; then you can proceed with the business of the assembly. Recess is often used when attendees wander out of the meeting room in the middle of a meeting and suddenly somebody notices that there aren't enough members in the room anymore.
Take other measures to assemble a quorum. You can, for example, appoint a committee to go make calls and round up enough members for your business meeting; while you're waiting for additional members to arrive, you can continue with the program or scheduled speaker. A motion to do something to obtain a quorum is treated as a privileged motion and takes precedence over a motion to recess.
Even if you have to make a decision about an urgent issue — the roof of the clubhouse has collapsed under the weight of three feet of snow, so you and the four other members who live nearby make the decision to call in the roofers — in the absence of a quorum, any action you take is at your own risk and not binding on the organization. If the membership doesn't agree that you did the right thing, or even if they agree but vote against a motion to ratify your action, you're "out in the cold," so to speak! In that case, the club doesn't have to pay the bill. You and your buddies do.
The motion to ratify allows the group to approve, by majority vote at a regular meeting (or properly called special meeting) with a quorum, your action and adopt it as the action of the group. After that happens, you and the others are off the hook, and your action is no longer null and void.