How do you handle multiple users in a web application?
User has registered and logged in to the site - so you need a table in your database to contain a list of users - and it would probably help if you had some unique ID in it to identify each user. You can then use ASP.NET session to keep track of which user is active in each session / whether logged in etc.
How do I add a user to Sendible?
So to add a new user to your account, just select the 'My Users' tab and then click 'New User'. When you're setting up a new user, you'll need to create the login details for them and also choose whether you want them to act as an Administrator, a Team Member or a Client.