1) If the group is small, the very best salutation is to use recipients’ names

Dear Carmen, Anita, Sean, James and Roland:

2) If all the recipients belong to the same group, use its name:

Dear Sales Team:
Dear Profile Committee:

3) If the recipients are in various positions or groups:

Dear Colleagues:
Dear Coworkers:
Ladies: (if all the recipients are women)
Gentlemen: (if all the recipients are men)

4) “Greetings” as a group salutation is neutral, succinct and not too casual, so always a good choice:

Greetings:
Greetings All:
Summer Greetings: (Suitable for a seasonal group announcement message.)

5) For more informal business writing, these salutations are warm and engaging:

Hello, everyone:
Hi Team,
Dear Friends:
Dear Readers,

Further Readings
http://www.instructionalsolutions.com/blog/bid/61960/Business-Email-Salutations-to-a-Group