Also strong recommendation: use command line git instead of GM's built-in workflow. It's easier to use (once you practice with it a bit), and if you get problems, it's easier to google the solution.
[Quick start guide]
To send your changes to the repo:
git add -A OR git add onlythehandfuloffilesyouwanttoaddbecausetheotherchangesaren'tdoneyet
git commit -m "This is what I did"
git push origin master
To get your friend's changes:
(close GM)
git fetch
git pull
(reopen GM)
I'm not sure if you need to close GM or if it'll automatically detect changes to the project on the disk, so this step might be optional.
If you're more than two-three people it's probably a lot more painless if you use a branch/rebase/merge strategy instead of pulling changes into the same branch locally whenever anyone else pushes changes, but let's not get too ahead of ourselves.