If they're all in a graduate program, Dear Student would seem obvious.
If the correspondence is to members of a body individually, perhaps asking each for his own opinion, then Dear Board Member would seem obvious, although that partly depends on what kind of membership of what kind of board. For members of the board of a company, Dear Director.
If the correspondence is to a body collectively, perhaps giving it your opinion, then Dear Members of Boardname might seem obvious; Dear Boardname would be better. Again, that partly depends on what kind of board. If it's a British limited company such as Bucks, Mega & Cashgrab then Dear Messrs Bucks, Mega & Cashgrab might be obvious.
Styles of address and salutations are very well covered in, for instance, Debrett's Correct Form and several big, church-bible sized variants of Webster’s Dictionary - but mine being stored in an attic since the WWW snuck up, I find the question too elusive for Google.