As you know one of the Scrum Principles is Self-Organising Teams. This means that, the teams should include all required roles to bring the product or feature to life. Also, all team members from the Scum Team must form a close and trusting relationship, a symbiosis in order to allow them to work together as peers. Shouldn’t exist us and them but only us.
Ir order to create this relationship and symbiosis they need pass through the four-stage model as described by Bruce Tuckman’s: 
In this stage, most team members are positive and polite. Some are anxious, as they haven’t fully understood what work the team will do. Others are simply excited about the task ahead.
As leader, you play a dominant role at this stage, because team members’ roles and responsibilities aren’t clear.
This stage can last for some time, as people start to work together, and as they make an effort to get to know their new colleagues.
Next, the team moves into the storming phase, where people start to push against the boundaries established in the forming stage. This is the stage where many teams fail.
Storming often starts where there is a conflict between team members’ natural working styles. People may work in different ways for all sorts of reasons, but if differing working styles cause unforeseen problems, they may become frustrated.
Storming can also happen in other situations. For example, team members may challenge your authority, or jockey for position as their roles are clarified. Or, if you haven’t defined clearly how the team will work, people may feel overwhelmed by their workload, or they could be uncomfortable with the approach you’re using.
Some may question the worth of the team’s goal, and they may resist taking on tasks.
Team members who stick with the task at hand may experience stress, particularly as they don’t have the support of established processes, or strong relationships with their colleagues.
Gradually, the team moves into the norming stage. This is when people start to resolve their differences, appreciate colleagues’ strengths, and respect your authority as a leader.
Now that your team members know one-another better, they may socialize together, and they are able to ask each other for help and provide constructive feedback. People develop a stronger commitment to the team goal, and you start to see good progress towards it.
There is often a prolonged overlap between storming and norming, because, as new tasks come up, the team may lapse back into behaviour from the storming stage.
The team reaches the performing stage when hard work leads, without friction, to the achievement of the team’s goal. The structures and processes that you have set up support this well.
As leader, you can delegate much of your work, and you can concentrate on developing team members.
It feels easy to be part of the team at this stage, and people who join or leave won’t disrupt performance.
This is why we should minimize any teams change since Its takes time to become a true team – a tightly unit with members who trust and support each other and who work together effectively.
Changing teams compositions makes the team-building process (four-stage model) all over again and in the end self-organisation suffer.
 Mind Tools Club | Forming, Storming, Norming, and Performing, Understanding the Stages of Team Formation
 Businessballs | Tuckman Forming, Storming, Norming, Performing model