What mail program are you using? It's possible that if you are using Apple's Mail program you might be able to drag it to the email but I don't use Mail so I'm just guessing on that. Otherwise you'll have to use Command+R to reveal the location of the file and then use your email reader's attach button to navigate to that location.
You can send an iTunes track in Mac Mail by clicking on the Attach button and navigating to your iTunes Library and selecting a track. I know you can send music you have purchased from iTunes to another Mac *you own*, but I don't think you can to anyone not authorized on your iTunes account.
Thanks for posting this question, I have been looking for the answer to it for a few days!
Thanks to Mike Osborn I believe I worked a way to do it, I've sent a music file back to myself which opened & played in iTunes & then sent one to my son-in-law who uses a Windows PC & his opened in iTunes too so I suspect the recipient needs that programme.
I used the Media window in Garageband to find the track file (in iTunes) I wanted to send (an amateur recording of a song, so no copyright problems) then dragged it into an open 'New message' Mail window. It was fairly short so wasn't compressed but it seemed to work OK.
Using Garageband was the easiest way to find the file for me but there may be an even easier way, I couldn't do it straight from iTunes but I might have been doing something wrong.