There is / There are

You question is basically the last sentence of your post:

"What is the right form to use for a pair of, a bunch of, a group of, etc.?"

I think the answer to this is a bit complicated, as it depends on the way the speaker/writer is thinking about the noun phrase. Is it in essence a singular entity, or in essence a plural entity? Thus, the examples given by previous posters are correct, but they each represent a different way of thinking:

The example given by Goos, "There is a bunch of bananas hanging from the tree," is clearly a singular entity being spotted by someone looking at it as such.

The example given by Michał Kosmulski, "there are a bunch of people outside," is different in that it is the people who are the real item here, not the bunch.

Thus, I would say that it is the focus of the speaker/writer which is the essential element here.