Either the group wins or the group loses. These are the only two possible outcomes of this discussion. Thus, the discussion and negotiation must be viewed as cooperative rather than competitive. All members of the group must be engaged in the discussion. No individual owns any given idea.
All ideas belong to the group.
Separate people from ideas. Try hard to understand the ideas that you're criticizing, few people appear so foolish as those who don't know what they're arguing against. Summarize and paraphrase the ideas you oppose, and ask other group members to see if you understand them correctly. Feel free to criticize and support the same idea. The best idea will emerge through reasoned discussion rather than through intimidation or suppression.