I read CS Lewis say that the Beatification is an "infinite good" and, hence, "infinitely outweighs the negations" (the stuff we shall leave behind here). He also argued, in Letters to Malcolm, that we shall not experience time as an infinite present because we are finite creatures and, therefore, can only experience what suits our humanity. Being that we are finite, does that mean when he (or other writers) talk of 'infinite joy' they mean it metaphorically rather than literally? In Weight of Glory, Lewis also refers to it as 'transfinite'.
Before I get to the point, I'm going to start with how we understand Scripture, even though your question is not about an exact phrase found in Scripture, because it sets a principle for understanding literature that is common not just to Biblical studies, but to other texts as well.
Next, I'm going to preface this with an honest "We really don't know, we can just give our best guess". We would have to ask the original author to know for sure, so I'm torn about whether to answer the question, or VTC as opinion-based. I'm answering because I think the question has merit, and I think that most would agree with the assessment I'm about to give.
Typically, "infinite joy" isn't taken as literal, it's taken as a figure of speech.
As explained in my answer on From a Fundamentalist standpoint, what does the phrase “Inspired, infallible, inerrant Word of God” mean?
The doctrine of inerrancy does not imply hyper-literalism. As noted above, in the second possible mechanism for Biblical Inspiration, we believe that God allowed the writer's literary style to be used in the writing of Scripture. Hyperbole (the use of exaggeration as a figure of speech. Example, "I'm so hungry, I could eat a horse.") is allowed.
This is a principle applied to any form of textual criticism. Determining the line between literal and allegorical can be summed up in a simple principle:
Dr. David L. Cooper, the founder of The Biblical Research Society out it much more simply. Dr. Cooper is known for his “Golden Rule of Interpretation” which is as follows:
When the plain sense of Scripture makes common sense,seek no other sense;
Therefore, take every word at its primary, ordinary, usual, literal meaning unless the facts of the immediate context, studied in the light of related passages and axiomatic and fundamental truths indicate clearly otherwise.
In this case, the question is, does it make sense that a finite being could experience infinite joy? I don't think so, becuase infinite implies that there wouldbe no joy in existence that a person doesn't have. I would have to have your joy, you'd have to have mine, and there would be in "individuals" We'd all be One. There's no corresponding Scripture to support the idea that we lose our individuality after death, so it just doesn't make sense.
It seems apparent that CS Lewis and the others who write of "infinite joy" are using hyperbole to express "a joy so great that we cannot imagine it" or perhaps "the most joy it is possible for a finite being to experience".