What does "Let's crash it!" mean? Let's join them and enjoy?

To crash or gatecrash a party or other social event is to attend without having been invited.

Interesting!
Thanks a lot.

BTW, does "plumb wack" mean "dumb guy" in "he's just plumb wack"?

"Plumb" is a colloquial intensifier; "wack" is colloquial and might mean "crazy" or "stupid".

"plumb" is the vertical analog to horizontally "level." A pole is "plumb" if it is, looking from any direction, at right angles to the earth.

So, plumb can mean something like "perfectly" or "straight-up" (completely). There is no doubt that the thing you're applying it to is "true" (as in "fits the category"). So, if, for example, ya say "that Babe is plumb fine," well, then, that means she's pretty damn HOT, see?

Same thing as a "true fine Mama."