Hi queequeg,
You can use Excel Data Forms which can modify and add data in a spreadsheet:
Highlight your current table range (Including field names/headers)
Goto Data->Form...
Click OK to confirm that the first row contains the headers.
and there you have your form. New records will be added to the bottom of the table.
Workbook sharing will work in the normal way: If two users attempt to save changes to the same cell, the first user will be able to save and the second user will be asked if they want to overwrite the first users data.
A better way to do this would be to create an Access database and link the spreadsheet as a table. This would allow multiple users to enter data simultaneously avoiding conflicts. Also, Access' form wizards can build you an input form in a matter of seconds.
Hope that helps,
Dave.