It's a bit late to do too much with them now since onions are sensitive to day length and yours should be trying to grow bulbs already.

What you bought were actually plants as opposed to sets that are very small dried onion bulbs. For plants, you needed to separate each of the plants in the 6-pack from each other and then plant the individual plants at the proper spacing.

My guess is that with 6 or so onions spaced very closely together in clumps, you won't get large onions on any of them just due to the competition. You might be able to pull all but 1 of the onions in each of the clumps and use them as scallions to give the remaining onion a better chance to grow a large bulb though it's getting late for it to size up now if it hasn't already started.