I like a guy at work... what to do about it?

I assuming you're a girl, and that he's straight and single, two out of three won't work.

Manoeuvre for alone time: "I'm going to grab some coffee, Tom can you help me?", there are a million ways to do this on the fly. Even after work, it's not a date, it's just grabbing a sandwich walking to your car etc. During these times if you're flirtatious, laugh at his corny jokes, touch his arm at appropriate intervals etc. (as a woman these are the tools at your disposal) he'll get the picture eventually without any awkwardness ensuing. If he's interested he'll respond positively, accept your standing a little too close, not be standoffish or uncomfortable when alone with you. This is when you can be fairly sure he'd be happy to attend a social occasion with you. If he doesn't initiate it then you could try something like "Hey Tom I'm heading for a drink with a friend after work, you should join us!", ideally your friend would be a taken woman, or a guy who's in on it and only stays for one or two drinks. It can be anything though, this is just an example.