I was wondering what that な is standing for.
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This 「な」 is a common sentence-ending particle used to casually conclude an impression/opinion.
"This is just my personal impression, but I think everyone is really enjoying his life."
Like many other sentence-enders, 「な」 all by itself could not be translated easily. You would only need to know it is used for a casual kind of conclusion.
An extension to the "casually concluding" な explained by @l'électeur
な adds the nuance/emotional quality of "I wonder" to the sentence, be it positive, neutral, or negative. Granted, the way the "wonderment" is added is not a hard and fast rule, and varies by context. Note that in the English versions below, the "wonder" may seem contrived, but it's only to illustrate the point.
For negative expressions, the "I wonder" can be a "sarcastic softener", as can be the case in English:
It's really noisy (and I wonder if you can turn down the noise for me, thanks).
I wonder if you can stop talking, thanks.
That's great! (I wonder if one day I might be able to have that too!)
(I wonder) if you can do that for me.
I was wondering to myself, "It looks like everyone's having a good time"
Thank you, (but with the following packed into it: "Sorry for making you go through all the trouble of doing me that favor. I wonder if (hope that) you won't hold it against me.")
Wow, that was cooooool. (I wonder how he/she did that?)
We might be able to understand it by comparing the one with it with the one without.
The latter implies that you have such impression by observing how actually they do something, while the former doesn't necessarily so. So, you can conclude that な stands for an impression formed through observation.