General:
* Unaware
* Unready
* Ill-equipped
* Unversed
* Inexperienced
* Uninformed
* Unpracticed
More specific:
* For a task/situation: Unorganized, disorganized, lacking resources, ill-prepared, deficient, deficiently prepared
* For a performance: Untrained, rusty, unpolished
* For a discussion: Uninformed, clueless, uninformed
* For a meeting: Unprepared, disorganized, lacking information
The best synonym will depend on the specific situation and the nuance you want to convey.