It is not advisable to add Users as Contacts within Event Temple. This can cause several issues, in regard to syncing your emails through the Email integration.

If you would like to include another User in an email thread, you could send the email from your email provider (Remember, the very first email needs to be initiated via Event Temple to track the communication thread.)

You could also CC them in the email from within Event Temple by typing in their email address and hitting enter.