Dear A__:

You wrote:
>a. There were Pete and Roger drinking at the bar.
>b. There were Pete and Roger at the bar drinking.
>c. There was Pete and Roger drinking at the bar.
>d. There was Pete and Roger at the bar drinking.

>Which of the above is grammatical?
>Could one add a comma before “drinking” in each sentence?
>
>
A and b are both grammatically correct and say the same thing, but putting drinking at the end emphasizes it more. A comma is necessary before drinking in b and d because it is out of the usual order and acts as an appostive.

A comma is not necessary in a or c but is normal. Basically, if you put a comma in, you are emphasizing the two men and “drinking at the bar” is a participial phrase modifying “Peter and Roger.” If you leave the comma out, you are putting more emphasis on the action, but this is less common, and could only be done with c for reasons stated below.

C and d are not grammatically correct since the subject is plural. However, this is the way many native English speakers say it.

You could argue that without the comma, c is OK. You could say that “drinking at the bar” is a gerund phrase and the subject of the sentence and that “Peter and Roger” is the subject of the gerund. This is a stretch, but one could make a case for it.

I hope this helps.