She is only there to help with communication. DON'T try to include the interpreter in the disscussion. She is not part of the group. DO make sure the lighting in the room is appropriate. DO talk to deaf person in a room with little or no noise.
Many deaf people have some hearing.
DON'T talk loudly to others, sing, bang the table, or make other distracting noises when the deaf person is trying to watch the speaker. DO talk to a deaf person in a room free of visual distractions.