I found out that my great grandfather was a Cape Rebel. I found the attached form your website, how do I find more information to confirm that Prisoner: 5541 was my grandfather Coert grobbelaar? He was born 18 Aug 1877 in Cradock Eastern Cape South Africa so the timeline fits if he was 22 when captured.
One option for finding more information on Boer participants is from the applications that some made for their medal but I don't think he did apply. The nearest match that I can find is a man called Grobbelaar, Burger Coert Petrus who served in Jeppe Commando. If you search the Forum for Grobbelaar in the title, you will see of the people with this surname who submitted applications.
I would also suggest contacting Rhino Research who are based in SA and may be able to identify other sources of information. Their details are in Contacts in the top menu.