Ridesharing from/to Somerville area with one other person?

[squirrelitude]

We are a family of two adults and a 4 year old. We're planning on renting a car, but I wonder if we could carpool with one other person—either in their car, or in one we rent. We pack relatively light (two frame packs, plus a little more for the kid) and I think we could manage another light-packer's stuff.

We would be doing the Friday to Monday schedule.