Just is modifying me here, not is
It’s just me.
In a comment, BillJ wrote:
In your example, it's a adverb functioning as a focusing modifier of "me". "Just me" is thus a noun phrase functioning as predicative complement of "be".
Here just is an adverb serving as a focusing modifier of the pronoun me. It works much like only does in this regard, which is another focusing modifier.
But just is still considered an adverb here, not an adjective. The same thing happens with nearly everybody and nearly finished: in both cases, nearly is still held to be an adverb even when modifying nouns and pronouns. If this idea seems strange, and I know it may, then this is because “adverb” is a junk-yard category that we throw just about everything into when we run out of names for word classes. It’s also because the simple parts of speech we teach third-graders in America are too narrow and simplistic for use in more nuanced, precise, and exacting analyses.
Boring technical details follow.
Boring Details and References
The Oxford English Dictionary allows that just can have many possible parts of speech, including not just noun, adjective, verb, adverb but also other several others like intensifiers and modifiers (including focusing modifiers) of both words and phrases. This can include nouns and pronouns as well as larger constituents like phrases and clauses.
The particular use mentioned by BillJ as a focusing modifier of arbitrary constituents falls under OED just (adverb), sense 6:
- Used to place the focus on a particular word or phrase.
a. No less than; absolutely; actually, positively, really. In weakened sense: neither more nor less than, no other than; simply, merely.
(a) Modifying noun phrases.
In just me, this places the focus on the pronoun me.
And That’s Not All!
However, just has many other uses as a would-be adverb. OED just (adverb) sense 2 shows just used as a modifier of all of these:
- quantifiers; adjectives; adverbs; adverbial clauses and phrases; adverbs introducing a subject or object clause; as or so with adjectives, adverbs, or quantifiers; nouns; noun phrases, noun phrases with the; prepositional phrases; pronouns; interrogative pronouns; conjunctions; correlative constructions using not just; and temporal clauses.
Specifically, it says it can modify all these types of things:
- As a modifier: exactly, precisely; actually; very closely. Also (now archaic) even just (cf. even adv. 5).
- a. Of place or position, modifying prepositional phrases and adverbs.
- b. Of degree and comparison, modifying as or so with adjectives, adverbs, or quantifiers: equally or quite as ——.
- c. Of manner, modifying prepositional phrases, adverbs, and conjunctions, esp. as, like. Also of reason or purpose, modifying prepositional phrases and conjunctions.
- d. Of amount, number, or quantity, modifying nouns, pronouns, and quantifiers.
- e. Of time, modifying prepositional phrases, adverbs, and temporal clauses.
- f. Of state, identity, or similarity, or of opposition or antithesis.
(a) Modifying prepositional phrases, nouns, pronouns, and adjectives. See also just it at Phrases 5, just my luck at luck n. Phrases 2f.
(b) Modifying noun phrases with the.
(c) Modifying interrogative pronouns and adverbs introducing a subject or object clause.
- g. In negative contexts in preceding uses of sense 2.
Summary
So just has numerous complex uses, virtually none of which can be usefully described by a simplistic part-of-speech assignment like “adverb”. It is possible that learners’ dictionaries fail to represent all these. However, if you have a good enough dictionary to consult, not an abridged one, you will discover that these nuanced uses and many are all described there.