You generally follow the rule book that you have chosen for your meeting. The bylaws for the group can override the rule book. No you cannot arbitrarily change the rules they must be voted on and there are rules for how many people must vote. Sometimes it's a percentage of the full body, sometimes it's a percentage of the people who show up to vote depending on how your bylaws are written. Some things require a majority vote, some things require a super majority vote. And everyone must be notified a certain time ahead of the vote of what the vote will be about. This is so they can't suddenly spring something on you when a small group is in attendance, but they still want to vote on it immediately. Sometimes you get an unethical chair person or parliamentarian and they can bully you into believing what they tell you unless you are very aware of the rules yourself.