Usage of "ever" in a negative statement

Solution 1:

In the example you have given, "ever" serves two purposes. It does act to emphasise the negative as you have suggested. It also adds a temporal dimension to the sentence.

If you consider your sentence without "ever" it gives the impression of a one time event:

I created a session but nobody joined.

Whereas your sentence indicates that this has been happening for a substantial length of time and implies that it is unlikely to change:

I created a session but nobody ever joined.

Solution 2:

Ever is a Negative Polarity Item (NPI), and is in fact a suppletive temporal form of any, the most sensitive NPI.

English has various NPI combined forms with any:

• She won't tell anyone/anybody.
• She's right about him, anyway/anyhow.
• I don't see her anywhere.

But there's no word *anywhen -- we use ever instead:

• *He hasn't met her anywhen.
• He hasn't met her ever.
• He hasn't ever met her.

Just like we use both instead of *all two, or went instead of *goed.

So it's an NPI time term, meaning "at any time", and it has a more variable syntax than anywhere does, since it preferentially precedes the verb, instead of being tacked on at the end.

• I don't see her anywhere.
• *I don't anywhere see her.
• I don't see her ever.
• I don't ever see her.

In the sentence in question

• I created a session but nobody ever joined.

ever is used exactly right. The negative trigger nobody (cf anybody) licenses the use of NPIs, and the NPI ever is deployed between it and the negated verb. In terms of time, nobody ever joined says that, in the time since the creation of the session until now, there was no person who joined the session that was created (note that the direct object of joined has been deleted, but is clearly implied). Which is what you intended to say, I gather.