It's amazing how one new participant can change the dynamic of an established group. We're often not conscious of it until after the group has convened, when we're either congratulating ourselves for wisely choosing the new addition or grousing about how things didn't quite click. No matter the outcome, it's best to be aware beforehand that new additions will play a role -- large or small; subtle or bold -- in the interactions and outcomes of group work.
Knowing that a new addition will be joining an established group, it's always useful to lay out expectations and ground rules with the individual in advance. During the meeting, group leaders will need to pay particular attention to providing more background information and insights in order to give the newcomer a chance to "catch up". Group leaders should also be mindful of ways to include newcomers in discussions by actively inviting their perspectives and posing questions for them to answer. A veteran group member can be of great help in shortening the learning curve by "buddying up" with a newbie.
Then there's the newcomer who exerts a negative vibe. How'd this person get on the team? And what can the group do to mitigate the disruption? I think most of the above rules apply, perhaps with a bit more steering exerted by group leaders, who can guide discussion by drilling deeper into issues, by seeking specific decisions from the group or by asking for additional information or deliberation to take place outside of the group. A private follow-up with the newcomer in this instance could bring to light new insights in the group's behavior that will be helpful when planning the next group interaction.
It can be especially hard for an established group to make room for a new addition. If the introduction is rocky, the group may struggle to establish equilibrium again. It will always take some effort for a group to absorb new talent and continue forward momentum -- the trick for group leaders is to facilitate regaining equilibrium as quickly and positively as possible.