Visit nexar.com and hit the Sign Up button. You register with an email address which when verified allows you to log in. On first login you will be invited to create an organization. From there you can create an application and the world is your oyster!
Yes, your colleague can invite users to their existing organization from their dashboard. You will receive an email which will take you to the sign up or login steps. Once you have joined the organization you will have same access to all the organization's applications as your colleague