There are a few instances where a user in Team Today might not have a team. In this instance you can easily assign a user to a new team.
You can another team easily and quickly in the company view
Reasons why a user might not have a team
A team may have been deleted that a user might have been a member of
You have domain registration turned on. This means any new users who are part of your company can sign up but won't be assigned a team. You can turn this setting on/off in company settings.
Finding an editing users with no team
Go to settings > Manage Teams
If there are any users unassigned to a team a message will be displayed at the top saying "Users with no team"
Tap "Assign users to a team"
You will see a list of users with no team, simply use the dropdown next to their name to assign them to on.