ArcGIS Online has a global shared name space for usernames
Ie, one global pool of usernames. The usernames must be globally unique, not just unique to your organisation.
Therefore you should never rely on any particular username being available. You should consider a username scheme that includes something about your organisation in each username. This would make it unlikely for the username to be already in use by another organisation or individual. ArcGIS Online does this for you initially when you enter the person's email address, it will take the part before the '@' and the first element after the '@' (before the first '.') and join them with an underscore. This is its default username scheme as in most cases, it will result in a username that has a person-specific component and a company-specific component.
Eg, from email@example.com, it would default to the new username of "jon.smith_acme".
If the username you want to use is not available, it is most likely in use by another company or another individual.
There is a geonet discussion about some of the other issues that this causes here.
Enterprise Log Ins
Alternatively, you can configure ArcGIS Online to use federated logins so that it authenticates using your own company directory services (LDAP or Active Directory). Then you will be using your standard company usernames and passwords (or email addressess and passwords) and don't need separate credentials for ArcGIS Online at all.
See, Organization -> Settings -> Security -> Enterprise Logins
You can even use a combination of ArcGIS logins and Enterprise logins for different users (or even have both types of logins for a single user).