Originally, CRO allowed virtually anything to be used as a username. Eventually, we grew too big to allow this, as multiple people were trying to use the same username across different companies. It didn't take long to realize we needed a better solution to keep this from happening.
In the end, we opted for the same solution virtually every other Cloud-based company utilizes for usernames: your email address! Much like your street address, your email address is exclusive to you. If you can log into your Facebook, then you can log into CRO Software!
If your company has its own website and domain name, then you can simply add them as: theirname@yourcompany.com. NOTE: the email address doesn't need to be active unless they will be receiving email alerts. If not, then the email address doesn't even have to really exist, as long as it's unique and in the system.