Adding colleagues as sub-users

Is it necessary to have a separate Site24x7 account before I add my colleagues as sub-users

The sub-users to be added need not create a new Site24x7 account.

When the primary account holder adds a sub-user, an email will be sent to the sub-user. So, when the sub-user accepts the invitation, a Site24x7 account will be automatically created for that email address. The sub-user can login using that email address.