Users can be in as many teams as you want. You can update the users manually in the Menu > Team Directory.
Alternatives to adding or moving users in bulk or automatically:
+ Spreadsheet import using the template on https://recognizeapp.com/resources and emailing it to firstname.lastname@example.org
+ Yammer user sync with groups - Company Admin > Settings > Scroll towards middle
+ Office 365 user sync - same as Yammer
Check the Resources page linked above for more information on user sync.