Often in business it is necessary to send an important company communiqué to a group of people, informing them of changes or revisions to company policies and procedures. If those people belong to a formal group, there are a number of options for addressing a letter to them.
Use the official designated name for the group if the group is formally organised and recognised by that name. If the group is Daughters of the American Revolution, for example, it is perfectly correct to address the letter to the group using "Dear Daughters of the American Revolution." This is the most formal way to acknowledge the information in the letter is for the consumption of the full group. However, it is doubtful that the letter itself will get shared with the group as a whole, but rather its officers and managers.
It is also acceptable to address the group by their collective title, forgoing the designation "Dear." You may begin a letter with "To All Account Managers" or "To Night Shift Supervisors."
If the group is one in which you would like to address an individual whose title and name are unknown, it is appropriate to use the greeting "To Whom It May Concern." While this phrase assumes that it is intended for an individual, it is appropriate to use when addressing a group. This salutation, while correct and appropriate, lacks the authority that comes from addressing an important letter with the correct contact person's name and title.
Use a simple designation such as "Distribution," or even "See Distribution," in the "To" field of a business memorandum, especially if the letter is an internal one. The names of all individuals on the distribution list should be completely disclosed in the letter, usually as part of the footer section. This list can be organised alphabetically or by organisation hierarchy with the senior managers' names listed first in rank order, followed by an alphabetical listing of all subordinates.
List the names, titles and addresses of all parties concerned if the group is small and not connected by association with a single company. This method is one of convenience to the recipients of the letter, who are provided with a handy reference guide to all parties in a single communiqué.