Research norms, looping in other researchers for ideas
This may be field dependent, but in my field (math), where groups of people that work on a paper together are rather small, you should ask all involved people before you enlarge the team. 

Note that there is a gray area, though. If you run into somebody and discuss your recent research and it seems like this discussion may be fruitful for the point where your team got stuck, it may or may not be a good idea to discuss your particular problem (depending on whether you know that guy before, there is a trusted relationship,…). When you are in this situation and are in doubt, you could resort to something like "It sounds like you could be the right person to discuss a particular problem we are having in our group. May I contact you later about this?" and then ask your group. In your situation, I would definitely ask the other group members before involving further people.