In this sentence, it means to join or to meet friends once you arrive somewhere. I would be careful with "hook up" because it had many meanings, and they all depend on the rest of the sentence. Some other meanings are "to know how to get a fair price", "to have a connection", or "to become physically intimate"

In your example "I'm going to hook up with friends" it means I'm going to meet up with friends. Did you ever hook up with any of your old friends when you visited your childhood neighbourhood? in other contexts it could also mean to work together with another organisation for a particular purpose: Micheal Jackson hooked up with Pepsi to promote his new album. You could also use the phrase to hook somebody up with something meaning to help them get something that they need or want: Do you think you can hook me up with some tickets for the rugby world cup? As Aaron said it can mean to start having a physical, intimate relationship with someone(normally casual).