The Scrum Team membership should change as needed, but not too frequently, as it may affect the team's ability to deliver value and maintain a high level of collaboration and self-organization. The Scrum Guide states that " Optimal Development Team size is small enough to remain nimble and large enough to complete significant work within a Sprint. Fewer than three Development Team members decrease interaction and results in smaller productivity gains. Having more than nine members requires too much coordination. Large Development Teams generate too much complexity for an empirical process to be useful. " 1 Therefore, the Scrum Team should consider the impact of changing its membership on its productivity and performance, and balance the need for stability and flexibility.
1: The Scrum Guide2, page 6
2: The Scrum Guide

Scrum
During the Sprint Retrospective, the Scrum Team discusses how the last Sprint went with regards to individuals, interactions, processes, tools, and their Definition of Done123.This includes methods of communication, the way the Scrum Team does Sprint Planning, skills needed to improve the Developers' ability to deliver, and the Definition of Done123.

The Sprint Retrospective is a time-boxed event for the Scrum Team to inspect how the last Sprint went and plan for improvements in the next Sprint. The Scrum Guide states that "The Sprint Retrospective concludes the Sprint. It is timeboxed to a maximum of three hours for a one-month Sprint. For shorter Sprints, the event is usually shorter."1 This means that the Sprint Retrospective should be held at the end of each Sprint, regardless of the project or release status. The Sprint Retrospective is an opportunity for the Scrum Team to inspect itself and create a plan for improvements, including to product quality and working relationships.1 Reference:
1: The Scrum Guide2, page 16
2: The Scrum Guide