Create a dynamic webform for collecting contact information

We have a joomla site which we would like to direct some of our customers to. The site needs to collect multiple contacts from each organisation we contact. An example would be a GP practice: We need the details of the practice, then below a grid for practice managers. Ideally the grid would allow adding as many as is required by having either a "add" option or alternatively it would adjust dynamically when a contact above is entered. Finally this is to be used for updating contacts and therefore if the contacts are within the database we need the existing details to be view-able but without login. To achieve this we would like to either enable population from URL or alternatively for the form to get the data base on a unique ID by running a SQL statement. I will be administrating this post completion and so would be interested in understanding the workings behind the form to make amends where necessary.