Is this an imperative sentence?

It's a complex situation.

Sentences like the presenting one are clearly intended to urge, if not impose, some kind of behavior on the addressee (though the addressee in this case is only a generic you, the same sense as one, but faluting a couple levels lower).

So in that they are like imperatives. However, it can be shown (as I do in my paper) that they aren't real imperatives, syntactically. They must be a different construction, mimicking an imperative. It's clear that the construction does have some conditional meaning --

• (If) you hang around with riffraff like the Weasleys and that Hagrid,
• (Then) it’ll rub off on you.

which is the beginning of a Modus Ponens syllogism:

• ((p Implies q) And p) Implies q | ((p ⊃ q) ⋀ p) ⊃ q

The second line is implied in context, and the conclusion follows.

An extreme case of this is

• Buy 10 and Save! (almost always with an exclamation point)

which means something like (boldfaced omissions)

• If you buy 10 [count noun]s, then you will save some money.

It could conceivably be read as an imperative (or, more correctly, an impositive as John Lawler explains in the comments):

[Go and] hang around with riffraff like the Weasleys and that Hagrid, and it’ll rub off on you.

But given the context, it's unlikely that the speaker meant it as a command. (After all - who would command someone to go get bad influences from riffraff?) It's more likely to be interpreted as a conditional:

[If] you hang around with riffraff like the Weasleys and that Hagrid, and it’ll rub off on you.