Users can only belong to one team at a time. They can switch teams at any time from their Profile, or account admins can move employees to other teams.
Every user in the account should belong to a team, so that they can see all relevant team and department goals, as well as specific Feedback and Review questions you've set.
For more senior employees and managers who may oversee multiple teams, we'd recommend you create a standalone team for that user under their relevant department. Because of the Line-of-sight function we use, managers can see all the data for their direct reports whether they're in the same team or not, and can set goals for their employees in any team. Department heads can also switch between teams on their Profile without changing any of their own goal alignment.