I know that "ever" can be used to express the strengthened negation but would it work like that?
I created a session but nobody ever joined.
Is it possible? It does not sound right to me. If not, what does this mean?
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:
Whereas your sentence indicates that this has been happening for a substantial length of time and implies that it is unlikely to change:
English has various NPI combined forms with any:
But there's no word *anywhen -- we use ever instead:
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.
In the sentence in question
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.
I also stumbled upon the tense. Since I created the session before holding it, I would have used the past perfect and written:
And, yes, there is a comma before but.