Email addresses can only be associated with one company on Mercury. If one of your team members had an account previously and wants to use the same email, you can still invite them with a modified email address.

• Click on your name on the bottom left and choose settings
• Go to Team and then choose Invite a Team Member
• In the email address field, add a modifier to their email address. So if they were using greg@gmail.com, you could send the invite to greg+yourcompany@gmail.com.
• They’ll receive the invite to their regular greg@gmail.com inbox and should be able to create an account as usual.
  
  Not all domain providers support this. Please check with your domain provider if you are unsure. If you are importing with Google and their email is already registered, you'll need to manually invite them in order to modify their address.