If you purchase Nelio Content Premium, the members of your editorial team in Nelio Content are the same users that you have in WordPress (those with at least the Contributor role). If you use the free version of Nelio Content, you can access with only a unique user.
To create a new team member just register a new user in WordPress. Remember that to do this you need to be an Administrator.
In the Dashboard, select the option Users, Add New.
There, introduce the information of the new user and select as a Role at least Contributor. Once you have everything completed, confirm and the new user will be ready to participate as a member of your team.