+1
How would you say it?

This was them (they) who were doing it.

These were they who were doing it.
+0
TicceSorry, man but I was not asking how it would sound better after having being been changed. You changed the sentences.
If you insist: These were they who were doing it (but it sounds terrible).
+0
Sometimes we change the sentences because the original sentence is so awkward that we can't imagine ever saying it. If I had been the first to respond, I would have suggested "They're the ones who were doing it." You did ask "How would you say it?" and neither of your original sentences is something a native speaker would say.
Students: We have free audio pronunciation exercises.
Comments  
TicceThis was them (they) who were doing it.
Simply "They were doing it" is best.
If you want longer, try
"This is the one who was doing it"

"It was those guys/girls who were doing it"
TicceThese were they who were doing it.
Again, simply "They were doing it" sounds best.
But you could say:
"These are the one who were doing it"
"It was those guys/girls who were doing it"
Sorry, man but I was not asking how it would sound better after having being changed. You changed the sentences.
 Philip's reply was promoted to an answer.
Site Hint: Check out our list of pronunciation videos.
 khoff's reply was promoted to an answer.
TicceThis was them (they) who were doing it.

These were they who were doing it.
This is a cleft sentence. Cleft sentences start with "It" -- not "This" or "These".

It was they who were doing it. (More formal than the following example.)
It was them who were doing it. (Also possible according to some authorities.)

The forms shown above are almost never used anymore in modern English. The modern form is

They are (or were) the ones who were doing it.

For a more extensive discussion see It's you who is/are answering.

CJ
these were they who were doing it
Try out our live chat room.
towel paint 283these were they who were doing it

Nope. Not natural English at all. Have you read the rest of the thread?