The key to interacting with a person with a speech related disability is patience. It is in no ones best interest to pretend you understand someone if you do not. Be patient and allow the person to complete what they are saying without interruption. Ask if writing may be easier than speaking. Allow the use of assistive devices such as speaking machines or computerized synthesizers.
Anxiety can aggravate a speech disability.
Do not insist that someone with a speech related disability talk in a group. Communication boards, symbols, and cards for commonly used words greatly aid persons who have difficulty with speech. Consider exchange of non-essential job duties, i.e. trade answering the telephone with other needed tasks.