movement limitation and security issues

I need some ideas for a project that I've got.

I have a spreadsheet with one column, containing user names. I don't want the users to be able to change anything in the spreadsheet - in fact, I don't want them anywhere in the workbook or worksheet except for that single column of names. Then, once they've moved to their name, I'll have them click on a button that will bring up a form, allowing them to enter their bid selections. Once that form is completed, the data will then be written to the sheet adjacent to their name and the spreadsheet will close.

I'm looking for ideas on the following subjects:

1. How to keep the users from changing anything and limit their movements to that single column of names?

2. How best to let the userForm (with bids) update the spreadsheet without changing any of the other security settings?

3. How best to implement allowing multiple users to simultaneously enter their bids?

Any ideas would be appreciated.

Bob