This is already basic functionality in the RE Events module. Is there something else you're trying to do?

You can do this. Click on the binoculars when you're entering the guest's name.

But this does not help when adding a spouse because she does not have her own record. I end up having to type in her name. This poses the risk of an unnecessary typo when the info is already accurately on the spouse tab of the record.

I think she means to link the guest's name during the transaction processing, so you don't have to go back into the event after you've completed all your transaction processing, look up the registrant and their unlinked guest, and try to link the guest. If that's what she means, then I like that idea too.