1. To engage in romantic/sexual activity with another at a party/gathering.
Usually, one or both partners are under the influence of alcohol, or another judgement imparing drug. "Hooking up" usually involves little or no emotional attachment.
2. An exchange. To give/recieve/share an asset, or something of value, usually something that is tangible.
"Hooking up" is most commonly used in reference to the sale/exchange of illict drugs.
1. Sally is always hooking up with guys at parties.
2. Sally is hooking me up with some crack later this afternoon.