For a project team to work together successfully and perform at their best a project manager has to put in some conscious efforts on team building. Even if some of the team have already worked together in the past. It is still important to rally them together around the current project, it's goals and constraints.
We are not talking about the go-out-of-town-and-have-a-day-of-activities kind of team building, since there will usually be no budget. But the team building that can be done with simple efforts as you are briefing your team on the project. Have a quick round of sharing your values, you start and then let your team each talk about their top 3 values. Then have a short discussion about how the individual values tie in with the company's values. Create a team charter in which you as a team decide how you will engage with each other. How will meetings be held, how will people prepare for them, how will people behave during meetings, will computers be open, will phones be on etc. what will team members do when conflict arises. It is very important that the team agrees on these things before the project starts up properly. This will help prevent conflict, disappointment with unmet assumptions and delays caused by this.