Only the Archmage of a fellowship can disband that particular fellowship. If this is done, it impacts all members of the fellowship, so it probably needs to be discussed and agreed upon by the entire fellowship first. It is done by entering the Fellowship Overview within the fellowship shield icon (upper left corner of the screen) and clicking the red button "Disband" button. Once this is done, it cannot be "undone", unless an individual manually re-creates the fellowship anew and manually invites the individuals back in (highly unlikely that each individual will re-join, but possible, I guess).
Alternatively, the Archmage can "pass on" that title to another fellow, if they (the original Archmage) decide to leave the fellowship or decide they do not wish to be Archmage any more. They would do this by going into the same shield icon (upper left corner of the screen), going to the Members tab, looking across to the Title column next to the individual that they would like to become the Archmage, and selecting that title from the drop down menu to the far right. Of course, this would probably need to be discussed and agreed upon before hand. Once this is done, the original person cannot "undo" the action because they will no longer be Archmage, and hence, no longer has rights to do such.
If one is not the Archmage of a fellowship, they always have the ability to leave that fellowship, but must wait the obligatory 24 hour "cool down" period before rejoining another fellowship.