Easy, you simply make the decisions within the confines of the shadow hierarchy that exists within all flat organizations.
Exactly! Business schools use a framework of formal power (job title, authority, etc) and informal power (influence and likability). The most effective organizations are the ones where the formal power structures are very close to informal ones. People know how to make decisions. Flat organizations end up being run by the loudest voices in the room. Ironically, flat organizations with largely informal power structure…
There's a nice parallel here to how parliamentary democracy arose. And conversely, there's no point having a chamber full of the people who you wish had power, if they don't have it.