From Wikipedia's article on intervention:
Rule 24(b) provides for permissive intervention, which is subject to the discretion of the judge hearing the case.
If the judge hearing the case denies a motion to intervene, can this (denial) be appealed in itself (to a higher court)? If not, does a denied motion to intervene still grant the would-be interventor the right to file an appeal on the substance of the lawsuit matter later?
A quick search shows that "interventions of right" (under Rule 24(a)) can and have been appealed, so I'm guessing appeal is possible for 24(b)... (Both the original plaintiffs and defendants are "appellees" in this 24(a) case I found, and the movant is the appellant.)