What is the proper title abbreviation for addressing multiple people?

The plural form of Mister is Misters, and the abbreviations Mr. and Messrs. respectively (although UK English drops the periods). The odd spelling is because "Messrs." comes from the French "messieurs". So your example would be phrased as:

Dear Messrs. Jones, Smith, Bloggs, and Flintstone

The abbreviation for addressing more than one Ms. is either "Mses."or "Mss."; note that the abbreviation "Mmes." (from the French "mesdames") is used for the pural of "Mrs."

If it's mixed between two genders, use the appropriate honorific for each set and join them with "and". So for instance:

Dear Ms. Smith and Messrs. Jones, Bloggs, and Flintstone

For addressing a married couple with the same surname, a variant may be employed.

Dear Mr. and Mrs. Smith