Step1
You have already noticed team members that don't seem to be part of the team, good for you! Now is the time to find out why these members aren't enthusiastic about being part of the team. Hold a company poll on how to improve team relations or ask the team members who feel isolated how to improve relations. Make sure to make it clear you are trying to help them feel more a part of the company and not trying to punish them.
Step2
Judging from the polls or discussions you had with your isolated team members, it is time to implement changes to the structure of the teams.
Example: Is the problem not being heard--whether suggestions or just being treated like they're not worth listening to? See if the team leader is at fault or another team member and talk to them about it. Tell the leader it is a part of his/her job to make all team members welcome. Start classes for proper handling of employees if necessary.
Example:
Team leader is highly critical of his entire team and acts like none of them are good enough--making it impossible for some members to deal with him/her.
Solution to this is simple, you need to make sure this team leader cleans his/her act up. The person might change his/her attitude in front of bosses to cover his/her activities up. Pay attention to complaints of team members and even have someone observe him/her that can do it without him/her knowing. Give this person proper training in handling employees--if the person still refuses to make the team a positive atmosphere do not allow him/her to be a team leader. It's a team leader's job to motivate the team--not to ridicule the team and make them ashamed to work there.
Step3
Make sure all team members are being given tasks that allows them to excel to their full potential. Nothing says, "I don't trust you to do a good job" like giving a team member a task way beneath their potential and not giving the member an explanation why. If it just needs to be done right away, tell him/her so and so forth.
Step4
Some team members are just better at doing independent tasks that assist the team. These people would feel more at home being given a task that requires them to work alone for a while. I know it's likely that some think this is odd thinking, but for some it is hard to just always force themselves to do everything with someone else. They like to contribute but like to work on their own unique piece that can only be theirs. They will actually feel more a part of the team by allowing them to do what they are good at and the way they can accomplish it. (Some actually accomplish more working alone than they can with the team--hint,hint).
Step5
Really respect the individual. I mean really do it. Individuals make the team and individuals have individual talents that make them unique. Forget the no "I" in team work unless you are talking who takes credit. Allow people to be who they are and contribute what they can to the team rather than forcing them to do everything a certain way in order to appease team work status quo. Congratulate individuals for their unique contributions to the team.
Step6
Just simply make everyone feel welcome period. No matter who they are, their social status, their behavioral problem, whatever--make them feel welcome. Don't pick out individuals to pick on based on anything. If someone wants to learn how to be a better team member, have a more experienced and well-trained team member help them rather than isolate anyone or make someone feel useless. Do not take away tasks from experienced people to give to newer people, rather have the new person assist and learn. Also, make sure to make the experienced person feel relaxed that their job is not threatened by newer employees. Never leave anyone feeling like they aren't a necessary part of the team.
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