If you want a colleague to register as a part of YOUR organisation, then invite them here: please invite them here.
People not in your organisation need to register under their own organisation. They can simply go to this website and register their organisation, and proceed to invite internal colleagues after registering.
In Facebook you register as yourself. In this website you register UNDER an organisation. Basically, People in the same organisation can see each others" messages and conversations. And they can send messages to other people, putatively representing your organisation. That's why it's so important to make sure you invite the right people to your organisation.