It's not the most intuitive thing, but it's /joinTeam. Observers get don't get assigned a team (team -1), whereas Team 0 is no team, and then you can be on team 1 or higher. Once you're on a team, you can reinforce units, and if you have a unit you shouldn't be considered an observer.
edit: Also, other players have to allow the change. When a player initiatives a /joinTeam, other players will have to vote to allow it with the /allowTeamChange command. Players should be prompted in chat to do this.