Just adding what everyone else has already said - you were normally required to have a full party before you could join any EX part.

2.2 Patch Notes
The following instances have been added to the Duty Finder:

Trials
Thornmarch (Extreme) / The Whorleater (Hard) / The Whorleater (Extreme) / Battle on the Big Bridge

2.28 Patch Notes
Players can now queue for Thornmarch (Extreme) and The Whorleater (Extreme) solo via the Duty Finder.

2.3 Patch Notes
The following duties, trials, and raids have been added to the Duty Finder.

Trials
The Striking Tree (Hard) / The Striking Tree (Extreme)

2.38 Patch Notes
It is now possible to register for The Striking Tree (Extreme) via the duty finder without a preformed party.


Oddly enough, this type of note was omitted from the 2.4 patch notes but if my memory serves me right that very same requirement was needed too.