A user is an individual (colleague or employee) designated to receive incoming calls on a Voxloud number.
Voxloud gives each user an extension number.
You can add as many users as you want to the Voxloud PBX, just log in as Administrator in the control panel.
If you are not permitted to add users, it means you have reached the maximum limit for the plan you are subscribed to.
You can, therefore, request an upgrade to a higher plan.