Does this mean board as in board of directors?
In which case why are they sitting on a staircase?
Yes, it means "a board of important people" (estro: leader; boss + -ar: a collective suffix, a group of people). They're sitting on the staircase because Dr. No and James Bond were having a "conversation" in the conference room :)
How would you say board meaning piece of finished wood, ready to be used in construction?
Tabulo, which is the same root used for blackboard (nigra tabulo).