Ask a question about being off the subject. Clarify the topic of discussion. Thank everyone for their input, but bring it back to the topic. Suggest that you're not sure where the group is. Ask the group to clarify where they are on the agenda.
Someone dominates the discussion. . .
Thank them for their point of view and ask for other opinions. Restate the ground rules about involving all participants in the discussion. Establishing agreement. . . Ask the group what topic they want to cover. Ask someone to summarize the position the group is taking on an issue.