Whatever you decide to do, keep the right to vote at a board meeting and the right vote as a general member completely separate. If you want to give all your (non-board member) parents the right to vote, make certain it is structured so that it is understood that they do not have the right to vote at board meetings. Otherwise, you'll have parents who don't attend board meetings, who don't have the details, who haven't educated themselves on matters, who don't participate in the discussions (which won't appear in the minutes anyway), having an equal say on matters of board importance. It will completely shut-down your board's progress.
Instead, you can place certain issues up for a general membership vote only after the board structures/organizes the details first.
Even that, however, isn't the standard among PTO's. Board members are generally elected in order to manage the issues on behalf of the parents -- not in addition to.
Our by-laws say that anyone (parent/guardian or staff) can vote, but specifically sets quorum to be a majority of the board plus 8 non-board members. I'm not aware of a requirement that quorum be set to a majority -- though Robert's Rules might say it is, if your by-laws don't say otherwise.
We did have to deal with a 2/3 requirement (state law) when amending our articles of incorporation. It would have been a little easier if we'd had more restrictive voting requirements, but if we had, I think we would have had to consider anyone who paid (voluntary) dues as a voting member, and that's probably 2/3 of the school, so it wouldn't have been that big a difference.
I think it would be a nuisance to have to track attendance to see who has been at N meetings, and to define the rules: does the count reset at the start of the school year? Can new parents not vote until N meetings into the year? Do you have to have been at the last N consecutive meetings, or can you go to 3, miss one, and vote at the following? What about people who arrive late or leave early (or just sign in and leave)?
Keep in mind that, by giving all parents (members) voting rights, you'll have to factor them in when it comes to whether or not quorum is met before the board can conduct any business. Quorum usually is either a simply majority (51% of all voting members) or is some higher number, like 2/3rds of voting members.
Unless you have an unusually active and involved group of parents in the school as an entirety, this by-law would create a nightmare for your board. Generally, visiting members are allowed to listen in on board business, in some smaller and less formal PTO's they may speak, but they do not have voting rights.
How many are on your board? How many are "liasons"?
