Bynaus is not a Federation member and this was explicitly stated in SCE: Belly of the Beast.
In fact no such thing was ever explicitly stated in The Belly of the Beast
or elsewhere, and another S.C.E.
story, 10 is Better than 01
, explicitly stated that Bynaus was a member.
The exposition when introducing the Bynars stated that while they were not Federation members they had agreed to abide by the rules and regulations therein for the duration of their stay on the da Vinci
I think I see the source of the confusion. You're probably remembering this bit from Fatal Error
: "The Bynar pairing weren't even Starfleet, they were civilians, part of an exchange program—although they had agreed to abide by all Starfleet rules and regulations." That just meant that 110 and 111 weren't Starfleet personnel, not that Bynaus wasn't a member of the Federation (though my use of "exchange program" might've been misleading). After 110 changed his name to Soloman and decided to stay on the da Vinci
, he formally enlisted.