More questions. Who decides? Do all active members have the same number and voting power? Or is there an elected council that makes membership decisions?
If you ever get the chance to watch the TOS ep "Journey to Babel", you might want to watch it. This ep partially deals with the matter of membership admission.
By the way, this episode reinforces the idea that Fed members are independent sovereign entities.
This is one of Kirk's captain's log from the ep:
"We have departed Vulcan for the neutral planetoid code-named Babel. Since it is in our sector, the Enterprise has been assigned to transport ambassadors of Federation planets to this vitally important council. The issues of the council are politically complex, the passengers explosive."
Each member planet is represented by an ambassador, which indicates that member planets are independent and sovereign in their own right. What Kirk said also implies that the Federation council is not really a legislature, in the sense of it being a national governing body.
With individual nation-states allowed to govern themselves on local matters within broad guidelines.
Perhaps a better way of thinking about this, is this: it is not so much that individual nation-states, or planets in this case, are allowed to govern themselves. It is the other way around.
UFP members are absolutely sovereign. The Fed is allowed to have whatever powers the members grant or yield to it. And for the Federation, the powers that it is granted seem to be generally limited to the areas of defense/security and diplomacy. And what is granted, could be taken back by the members.
Actually this episode is the best argument that the Federation members are sovereign states (and that I'm wrong lol). An "ambassador" does imply that they are representatives of a sovereign government.
It may be that early on in the series they were still working out the background details (ie Starfleet Control, UESPA, and what the Federation is exactly). But you'll notice in later episodes, Sarek is called Ambassador Sarek, or Federation Ambassador more often than the "Vulcan Ambassador". This would be more accurate because if he only represents the Vulcan government he cannot possibly negotiate a treaty between the Legarans and the entire Federation. Of course, he can still be called Vulcan ambassador, as in hes an ambassador who also happens to be a Vulcan.
I will admit though, if you only look at TOS, episodes like this make it a little bit less clear. But if you look at the franchise as a whole, to me, I think it's pretty clear. I mean the Federation Council can order its citizens on Dorvan V to evacuate over the objections of the Dorvan V leaders. The Dorvan V representative gets to have a say at the Federation Council, but once the council votes, the decision is final. If you don't evacuate, Starfleet security officers can come down and remove you by force. That to me is pretty clear that the federal government is boss, much like the US, Canada and every other federal republic in the world