A few days ago, I came to my desk to find an envelope. Inside of the envelope was a GroupOn for a pizza place located near my office, with a note on the back from a coworker thanking me for my help and support. I've worked with this coworker for about a year, and in the past few months we've started to work more closely together as he helps with a certain aspect of my product line (which was an entirely new area for him). We've also gotten a bit closer as friends, as each of us has had other coworkers whom we've been close with leave our organization.
I immediately went to his office and thanked him verbally, and I most likely will use the GroupOn for a lunch with him. Is a written thank-you note also necessary? Would it be awkward to invite other coworkers if we go out? The coupon is good for $20, which would cover 2 people, but probably not 3. CW and I have lunch out together fairly regularly (maybe once a week), sometimes with other coworkers, sometimes without.
(He has a girlfriend with whom he lives and whom I've socialized with a number of times, so I doubt there's any romantic hopes on his side for this.)