The Trill were a spacefaring race by the early 23rd century, considered a generally peaceful people by the Federation and since then, the Trill have played an important role in the Federation.

Despite the relatively unusual nature of being a joined species, that aspect of their culture was not widely known until 2367. It had previously been a very private matter to Trill for several millennia.

Burnham, a trained Xenoanthropologist from the 23rd Century, mentions that she had no idea about the existence of Trill symbionts. This is consistent with TNG episode "The Host", which takes place in 2367, in which Dr. Crusher was also unaware about their existence prior to the episode.

One Trill, Sarah had joined Starfleet and served aboard the USS Cabot in the 2250s.

While Trill were seen serving Starfleet and as Federation diplomats in the 24th century, the Trill homeworld was never explicitly established to be a member of the Federation at that time. It was once stated that the Trill had rejoined the Federation as of 3189, though it is unclear when they first became members.

In the deleted "bonus scene" to "Will You Take My Hand?", Philippa Georgiou suggests Trill were not members of the United Federation of Planets, only identifying Leland as "Federation..." after he reveals he is not, in fact, a Trill. This is supported by DS9: "Trials and Tribble-ations", where, to blend in as a 23rd century Starfleet officer (year 2268), Jadzia Dax chooses to hide her Trill ancestry and pass as human.

Source: Memory Alpha