It can be difficult for a functioning or dysfunctioning team to bring in a new team member. There is a period of adjustment that has to occur for both the team and the new member to the team. Teams traditionally go through four stages of team development; forming, storming, norming and performing (Tuckman, 1965).
Team members will reluctantly, if at all, accept a new member to their team until he or she has proven they belong there and the first two stages (forming and storming) have adequately been worked through. Leaders can do some things to speed this process up and help acclimate the new member to the team.