Just keeps talking: Interrupt tactfully. Ask a question to bring him or her back to the point being discussed and thank them for their contribution.
Seems afraid to speak: Ask easy questions. Give them credit to raise their confidence.
Insists on their own agenda: Recognise the person’s self-interest. Ask him or her to focus on the topic agreed by the group.
Is just not interested: Ask about their work and how the group discussion could help.