Allow multiple users per organisation #293

As a potential user
Given someone is already signed up for my organisation
When I try to sign up as a user for my organisation
Then I should get a message telling me that an email has been sent to the organisation admin
And the organisation admin should be able to add me to the organisation

As an organisation admin
When I get an email telling me that someone want to join my organisation
Then I should be able to add that person to my organisation