please have a look at the following sentence:
If there's any trouble with the captain, you're all with me!
My question is: what word class/sentence element is 'ALL' in this sentence?
My guess is that 'ALL' is a pronoun and as such functions as subject predicative (subject complement), 'WITH ME' being a modifier of 'ALL'.
"All" in your sentence is a pronoun -- an "indefinite pronoun". A list of the indefinite pronouns can be found on a chart at the "competitive sentence diagrammers" website.
It is the predicate nominative in the clause, modified by "with me".