Send email invitations to your colleagues or let them signup directly to your company account with a shared email domain.
To invite your colleagues, go to Settings and select Team from the sidebar. Enter an email address of your team member and click Send invitation:
Your team member receives an email to sign up and will automatically be added to your company account.
Let team members sign up independently
Below the invitation form you find the option to Allow people with @yourdomain.com emails to join your team automatically. If this is active and you are using a company email address (i.e. @mycompany.com and not @gmail.com), other users with the same domain can sign up without invitations and are added to your account.