"Dear" when addressing an audience, implies a degree of personal or relational bonding which is closer to far closer than would be found amongst the general public or members of a workplace or interest group.
It may be used amongst old friends of long acquaintance - but even then would be somewhat formally-informal.
It could be used almost without notice when addressing friends of the deceased at a funeral (even by a speaker who was not one of the group), among people joined in an emotional cause and similar. Possibly used in a 'trade union' type context, sit-in, protest rally and similar where people are joined in a common cause. eg a "Occupy Wall Street" or similar speaker may address the protestors with "Dear friends ..." even if they were all strangers. Emphasises the close personal relationship or common-cause aspect of the occasion.
However, even where it was usable, omitting it would not be noticed. Using eg "Dear friends ..." seeks to add emphasis to the closeness of relationship, but omitting it and saying eg "Friends ..." in no way suggests that the relationship is less than close - so not using "Dear" is by far the safer option.