Learn English – Which one is grammatically correct: “one of the great” or “one of the greatest”

This is correct, as for one references back to ideas. See here:

People often say “for one” when they mean “for one thing”: “I really want to go to the movie. For one, Kevin Spacey is my favorite actor.” (One what?) The only time you should use “for one” by itself to give an example of something is when you have earlier mentioned a class to which the example belongs: “There are a lot of reasons I don’t want your old car. For one, there are squirrels living in the upholstery.” (One reason.)

Additionally, as I commented on the question, it would be

For one, I had mainly been...

You have several options. I am repeating some of the other answers because it seems practical to have them all together: I hope my plagiarism will be forgiven!

1) Five people went to the party last night, with me being one of them.

This is common and accepted, though not by all purists: it does look a bit informal to me (mostly owing to "with"), which might be fine in the context of a party. "Being" would be a participle; the construction would be called "fused participle", or "accusative with participle" in classicist terms.

2) Five people went to the party last night, with my being one of them.

This is how "me" would traditionally be expressed; but "with" still looks informal, which contrasts weirdly with traditional "my". "Being" would be a gerund here.

3) Five people went to the party last night, I being one of them.

This would be the classical absolute construction. It is impeccably correct, but it sounds rather stiff, especially in this context. "Being" would be a participle.

4) Five people went to the party last night, with myself being one of them.

I know this construction exists, but using "myself" this way might result in some criticism. This would certainly not be my choice.

5) Five people went to the party last night, and I was one of them.

This looks much cleaner. Why use complex constructions in this context if you don't need to? The version with "...; I was one of them" looks OK as well.