Was / Were

[jack]

Are these correct? If not, why?
1. Imagine John and Susan that were there.
2. Imagine John and Susan that was there. (If this is incorrect, why? 'John and Susan' cannot be view as one thing right? Because they are different people?)

[Casiopea]

Try, subject verb agreement,

1. Imagine that John and Susan were there.
2. Imagine that they were there.
3. Imagine that she was there.
4. Imagine that he was there.