(written from a Production point of view)
Star Trek Interstellar: The Official Star Trek Fan Club was the first, what founder Gene Roddenberry had coined "officially authorized" fan club for the Star Trek franchise and was established in 1968. Initially, Ruth Berman edited the club's publications and co-ordinated the fan club on behalf of "Star Trek Enterprises", Roddenberry's merchandising company – and actual operator of the fanclub – that was later renamed "Lincoln Enterprises".
Actually, the fanclub and its associated fanzine Inside Star Trek were neither "official" nor "authorized" as then actual owner of the Star Trek brand, Paramount Pictures, has never officially issued an endorsement or authorization, implicitly conceded as such as the studio was nowhere mentioned as the copyright holder in any of the club's publications. Having acquired Desilu Studios and its assets – including Star Trek – the year previously, the new owner at the time was not in the slightest interested in their recent Star Trek purchase, and was actually looking for ways to cancel the series as soon as possible. As a result, no commercial or publicity activities on behalf of the series were undertaken by the Paramount Publicity Department, and those that had been, such as the free mail-order distribution of publicity photographs to fans, were immediately scrapped upon the acquisition of Desilu, the resulting slack immediately taken up by Star Trek Enterprises, who continued to do so through the fanclub on a commercial basis this time. The moniker "officially authorized" therefore was not only illegitimate, but also a pure flight of fancy on Roddenberry's part and at the most to be taken as "officially authorized" by himself. As was customary at the time in the motion picture industry, the moment Roddenberry sold his Star Trek is...-pitch to Desilu in April 1964, he signed over all rights and title to his creation, save for his "created by" credit, with financial compensation for work actually done to be individually determined on a per production contract basis.
The fan club published twelve issues of Inside Star Trek and enjoyed close access to Star Trek: The Original Series' cast and crew during the production of the series' third season. Following the show's cancellation by NBC in 1969, both the club and the magazine were discontinued, reinforcing the fact that they were anything but "official".  Still, considering the contemporary closeness to the actual production of several of its contributors (Roddenberry was still the series' Executive Producer at the time, albeit in name only for its third season), and the mere fact that they were the very first ones, both club and fanzine are for all intents and purposes considered "official" by the Star Trek community at large.