If, as a multiclassed wizard/druid, I have an imp familiar from the find familiar spell and I wild shape into a rat, can the imp hold me and turn invisible?
The Invisibility ability the imp can use reads like this:
Invisibility. The imp magically turns invisible until it attacks or until its concentration ends (as if concentrating on a spell). Any equipment the imp wears or carries is invisible with it.
Your character isn't equipment so the imp can't make them invisible as well. However, if you hid inside of a bag and the imp was carrying the bag it would become invisible and you'd be effectively invisible as long as you stayed inside of it.
As noted by Tiggerous, wizards by default cannot have an imp familiar. The Find Familiar spell states:
You gain the service of a familiar, a spirit that takes an animal form you choose: bat, cat, crab, frog (toad), hawk, lizard, octopus, owl, poisonous snake, fish (quipper), rat, raven, sea horse, spider, or weasel. Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast.
Imps are not on that list and they are not beasts either.