Weelllll... Earth is the capital of the Federation, among many other indications that Earth is a Federation member, so...
As far as canon is concerned, Trill's status as a Federation member world is just never addressed. No Trill ships are ever seen, this is true, and when Dax is training that Initiate, she suggests Starfleet as a potential career move, and it's presented as a "standard" move.
It's possible, but not likely, that Trill is a non-member protectorate of the Federation like Bajor, perhaps in a similar situation as Puerto Rico for the United States. The Trill we see in Starfleet might then be those who've chosen to go that route rather than the Trill equivalent, like Bajoran refugees Ro and Sito. Why Trill wouldn't join, who knows? Memory Alpha's got a really interesting talk page about this issue.
For what it's worth, TrekLit and the DS9 Relaunch present Trill as a standard Federation member world, and that's my personal opinion on the matter.
With three of the hosts as being Starfleet officers and with Curzan Dax being a Starfleet Ambassador it looks to be very clear to me that Trill was a member of the United Federation of Planets. Unless anyone would believe that President Obama of the United States of America would send an ambassador to help with a peace treaty from say Canada and a citizen of Canada as one of the primary American ambassador to a different foreign country. With so many posters pointing out that Curzan Dax was a Starfleet Ambassador, without thinking how politically incorrect it would be if President Obama did almost the same mirror move.
Also, unrelatedly Evil Ezri, where in East TN are you from?
You know the city that has the Neyland Stadium? Hmmm talking East Tennessee, let me get you a can of coke, than gives you a Mountain Dew. Now if you’re an old man, let me get you a can of dope. You are welcome Bud.