Marathon Sports does a weekly run group that is awesome for people who are mostly looking to have a good, casual time. They have a few locations--I used to go to the one in Brookline in Washington Square.

I'll also second Pioneers Run Crew and also throw in Forest Hills Runners as a very welcoming group.

If you're looking for a casual 5k for runners and walkers of every ability, you can go to Jamaica Pond parkrun or Danehy Park parkrun, which happen on Saturday mornings at 9 AM almost every week of the year. Totally free, and they're timed.