The Organizations tab lists every requirement as a tick or a cross — region, relations with members, economic size, defense share and so on. Meet them all and you can apply; if you are turned down, wait a while and try again.
You pay daily dues and you have to keep the obligations. Slip and you get a warning first; if you don't fix it within a year your membership is suspended, and while suspended none of the benefits apply — not even the defense pact.
A member tables a proposal, the bloc negotiates for a year, and if support clears the threshold the resolution binds every member for five years. You vote openly, and you can see the estimated support before you table anything.
Some blocs protect their own goods: members hold part of their output in the vault, sales outside the bloc carry a markup and sales between members get a discount. A resolution can close that good to outsiders completely.
Leaving isn't one signature: the process runs a year, you keep paying dues and the benefits stop working meanwhile. At the end you pay compensation that grows with every year of membership — a long-standing member can't walk away cheaply.