As guest implies copyright is not a verb. You can not copyright anything and neither can Paul Simon. To use it in this context only increases the misunderstanding of what copyright is. Copyright is the automatic posession of a "work" by it's author at the time of it's creation. Any reproduction of that work requires consent of the author.
The consent and/or the right to copy or publish may be legally given or sold to others or it can be stolen. Often the registration of the copyright with a government agency is mistakenly used as a verb , but the registration only establishes a date of claim to the work's posession.
It is common for music publishers and performing artists to slap a copyright mark on someone elses creation. This is a form of theft but often the burden of proof lies with the true author, who dosen't have the financial resources to fight a court case. Music publishers and recording companies are usually both pissing in the same pot.
If you wrote it the copyright is yours . If not it is under someone elses copyright or public domain. Scarborough Fair is public domain and any claim otherwise is bogus. That however, does not prevent industry attempts to claim any fees involved.