From the Users page under Account Settings, you can click on the Import Users button if your data source supports importing users. Right now, the following connections support user imports:
The import will pull over all users from your source. You can configure users as Viewers, Data Editors (if the connection is enabled as two way), or Designers.
If you have a persona set up on your data source, the users will automatically be matched up against the user name and email values. For example, if you have a Basecamp Team Member persona, the Basecamp import will match users to that persona and set their data level security permissions appropriately.
You can either set it up to send an introductory email to each user, or to just import the users without sending out any sort of email. Sending without an email can be useful if you just want to import users for purposes of then sending out recurring report emails.