Depends on what you mean by "digital signature".
Here, we consider authentication into the workstation to be adequate. Hence, we use the user's login (enforced by Active Directory) as "proof" that the user is who he says he is. A script records the user's login credential (account name), date, and time on the record as a "signature".
Other options might include the use of a tablet and pen, with the content inserted into a container field, or the use of an iPad with similar functionality. You can encode geolocation and other data into the image thus stored.
Mike's way is more secure...and I use that when practical
I do find one particular situation, however, when one person has to "sign off" (i.e. 'witness') another person's work...and it's a pain in the butt to relogin - sign - relogin.
I thus have a Password table in the solution just for that one type of signing.
The "signer" clicks the field to be signed which launches a script...asking for the password which is put in a field through the custom dialog...which then relates that record to the password table...then the script sets the signature field with the name stored in the related password table (+ Timestamp). The script then wipes the password from the field before ending to break the relationship.
This is a less secure method than the native authentication of AccountName, but is much more time efficient than leaving one account to log in under another, and then back again.
If "signatures" can be done in the signers own account..use the accountname as Mike suggests...it is a more secure way of doing it.
As Mike mentioned, if your users are running FileMaker Go, then you can record their signatures and store them in container fields. It's a really nice feature of FileMaker Go.
If your users are running FileMaker Pro, then you have other options. One option is "FMEasySignature," an open source signature capture solution that I released late last month. It's still in beta, and the current release doesn't support Windows (although I'm working to resolve that). In any case, you might want to check it out: http://fmeasysignature.com
Good luck with the project!