If I setup a separate account for each family member how do I link each account to the TurboTax Deluxe software I already paid for?

When you use Online TurboTax, you get one return per fee.  Each return needs its own account and user ID.   If you use the same account and user ID for a second return, you overwrite the first one and lose it forever.

https://turbotax.intuit.com/personal-taxes/online/

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

If you have multiple returns to prepare for various family members, desktop download software would be more cost effective.

 You can prepare and  e-file up to 5 Federal returns—more if you mail the additional ones.  If you have multiple returns to prepare for family members, Desktop is more cost effective.

Desktop --unless you specifically buy federal  Deluxe only from a retailer-- will include the software for one specific state.   You can prepare as many returns for that state as the number of federal returns you prepare.  E-filing state returns is an extra fee which can be avoided by printing and mailing the states.   Additional--other--state software is an additional fee for the software.

No, the online programs only allow the user to file one return per account. Each user must set up his own separate account. The download version however allows up to 5 returns to be filed per program.

for online a separate account is needed for each taxpayer.   A unique ID is required but the same e-mail can be used

if you use one of the desktop apps, it's probably cheaper. One fee covers the federal and one state return but there is usually an additional fee for e-filing state. You can do as many returns as you want with no additional fee. You can save each of them. Turbotax automatically uses the taxpayer's name. You are limited to e-filing 5 returns. That's an IRS limit not a Turbotax limit.