The Ypsilon Pact is governed as a Senatorial oligarchy.
The Senate holds all powers within the Ypsilon Pact, including that of defining its own purpose and membership. In general, the Senate will consist of the active core of the alliance, with some rotation happening based on interest & activity. New Senatorships are granted regularly - the size of the house is not fixed - but they are also suspended for inactivity quite quickly. That way anyone on the list of current Senators can be expected to be politically active, and if they are not, they will soon cease to be on the list.
The Senate Chair, DoctorPC, founded the alliance and today works as a public representative of the Senatorial views. As such, the Chair only votes in the Senate to break a tie that could not be broken in any other way. That way the Chair can always act as though they agreed with the Senate, whatever their personal views may be. List of YP Senators can always be found as the "topic" of our chat, #ypsilonpact-public.
How to join? In principle you need to be invited by a Senator and then follow their instructions to get through the full application process. But worry not, you can simply see who your nearest Senator is and express your interest. They will be glad to consider you as a candidate and walk you through the process.