If the new user is you, or someone in your organization: 
Your Organization Admin needs to create an account for the new user and give them access to projects. If you’re already using Aconex, find out who your Organization Admin is here. If your organization isn't using Aconex yet, your organization needs to register to use Aconex first.

If the new user is someone from another organization:
Their Organization Admin needs to create an account for the new user and give them access to projects. If they already have an account you can invite them to your project.