It will really really depend on the organization in question. Most anarchist actions in my experience are done by small interest groups of between 5 and 20 people. With this size of a group taking a vote is not normally needed. A conversation between everyone is had (sort of like a business meeting) and a consensus is reached. And this is not some formal consensus, more like the consensus that is reached when a group of friends is deciding on where to go eat or what rules are going to be used for a pick up game. If some people are adamantly opposed to this decision then the group may decide to accommodate them if it is not a major issue for everyone else, or that person can choose not to participate in that given action -- no hard feelings, they'll just catch you next time.

Now, optimally, large organizations are made up of such cells as described above -- with a particular cell creating and leading an action, and people who want to participate in that action freely and voluntarily participating. For example, a small group may get together and create some propaganda or organize a book fair (or something more exciting) they can then let a larger group of people outside of the cell what is being done, and people can join up if they want.

Sometimes coordination demands a decision made by and for a large group of people -- honestly though, I have never personally experienced being part of something so large that a conversation could not decide things. Except for Occupy, but group decisions there were largely a train wreck fiasco -- at least in my area.

You may want to check out the way the Zapatistas make decisions though. I find that very anarchistic. Basically they use a version of family/clan/interest group based representatives creating action committees. Seems to work really well for them. My favorite anarchist theoretical books, like Bolo Bolo, paint a similar picture.