Too many teams have boundaries like that softball team. It’s never clear who will show up, and who the team can count on to play. If you care about getting work done, don’t mess with team membership.
The fundamental criteria for a team is stable membership. To be a team, people need to know who the team members are. Weak membership boundaries make it hard for any team to do it’s work. Unstable team membership leads to miscommunication and misunderstanding. And that adds up to time and money. Team members can’t be mutually accountable if they don’t know who is makes up “we” when they commit to deliver.
Some organizations assign people who in between projects (or managers) to what ever team seems (operative word “seems”) to need more staff. This is the warm-body theory of staffing, that holds that you can plug a person with roughly the right skills into a team and achieve productivity right away. (There’s a related fantasy, that full utilization is desirable and efficient). The reality is that plugging people into a team creates a revolving door effect and the effort to bring people up to speed slows down productive work (see Brook’s law).
In terms of getting work done (and keeping people engaged) organizations are better off it people who are “in between” work on their own special projects, attend training, or just take a breather.
The belief that a group can start working at full potential the first week they are together is a fantasy. It’s also a fantasy that teams can function with who ever shows up.