Need help with a sign-up form.

[Madeleine Morris]

I am in need of some help making a form for multiple people to use.
It's a work shift sign-up form for people to sign up to work for a single 2-day shift.
But I don't want more than one person working at a time.

I want to create a sign-up form for up to 5 people to use online simultaneously. So they can choose their dates, first come first served, and no overlap, and they can see when everyone else is working.

I have group boxes with option buttons. Each person has their own group box and can only select one of the options.
I edited the text for each option to show the pairs of dates available to them.

How can I prevent people from selecting options in other people's boxes?
How can I prevent people from selecting overlapping days?

Under format control, I linked the option for each box to a cell. Maybe I can set rules for those values to give someone an error message if they choose an option that overlaps with someone else's.

The alternative I was thinking was data validation for checkboxes next to the pairs of dates. And then each person is given cell-editing permission for only his or her cells.
That prevents people from editing others' dates but I can't figure out how to prevent overlapping dates that way.

Example:

Sansa, Tyrion, and Arya are working in June.
Sansa is allowed to work any of the following pairs of dates: 6/8-6/9; 6/13-6/14; 6/18-6/19
Tyrion is allowed to work any of the following pairs of dates: 6/9-6/10; 6/14-6/15; 6/19-6/20
Arya is allowed to work any of the following pairs of dates: 6/10-6/11; 6/15-6/16; 6/20-6/21

None of them is allowed to work any other dates than the ones designated for them above.
No one is allowed to work on the same day.
First-come first served, so if Sansa comes first and picks 6/8-6/9, I need to prevent Tyrion from being allowed to select 6/9-6/10.
They have to work 2 days in a row so they have to make a single selection of a pair of dates.
I don't want them messing with each other's selections.

[Madeleine Morris]

I have an alternative option to using group boxes and option buttons: dropdown lists

I could require each person to select their dates from drop-down lists.
But then how would I prevent overlap?

[Melvosh]

If you could upload an example workbook, it would be helpful. Make sure it includes sample data and expected results. To upload, click Go Advanced (below the quick reply box), then Manage Attachments (scroll down a little). Click Browse to find your file, click Open, then click Upload. Click Close this window, and your file will be attached when you submit your reply.