How do you sign for someone else in their absence?

Not unless they have that person's power of attorney to do so.

they can but not if its on any kind of document that needs you to sign it personally.otherwise its fraud. Added Info. Perhaps with a Power of Attorney document, properly notarized and all nice and...

Yes, if you hold a valid power of attorney to act on their behalf. Otherwise no. Signing for someone you do not hold power of attorney for is fraud, and against the law.

No, in some cases it is illegal.

Not unless you give them your password or someone finds out what it is.