From Resources > Users > Internal Users (tab) click + New

Enter mandatory fields:
Alias (Also known as Resource Code *Only required if not system generated)
Name, Surname, Title
Email 
Profile (*Profiles available when created based on Admin Access, Full Access, Project Control Access or Standard Access - Internal. For more information on Profiles click here)

Add optional information where applicable:
Manager *Used with Teams
Organisational Unit *Will only appear if Admin > Configuration > Company Preferences has Activate Organizational Units set to Yes
Skills

Hit the OK button at the bottom of the screen to create the Internal User.

Within a few minutes of the user is created they will be sent an email from info@hydra.cloud which looks like this: