It all depends on how legally binding and court-safe your user's digital signatures have to be. There are various levels of "quality" of the digital signature.
The lowest level is the self-sign signature, which your users can set up themselves. With this signature, the signer claims to be the signer he claims to be… but if you are certain that you never have to proof anything about that signature in court, that is the easiest and cheapest way to go ahead.
The "better" digital signatures use certificates and credentials issued by Digital Signature providers and Certification Authorities. In such cases, the user may have to present himself at an office of the Certification Authority with according documents (passport etc.), in order to get the according credentials. Often this does include a signing terminal and a personal signing code.
(note, here we talk about serious stuff)
Adobe recently acquired a Digital Signature system provider; EchoSign. I am not familiar with their system and "quality". But it might be worthwile looking at what they offer.
In any case, there is no "digital signature without the user having to do something beforehand".
Hope this can help.