How do I merge another developer's branch into mine?

You first need to add the other developer repository as a remote.

git remote add otherrep uriToOtherRep

Then you fetch changes from there

git fetch otherrep

And then you merge the branch from the remote repository into yours

git merge otherrep/branchname

Happy merging!

You can also do "git pull", it'll pull the changes of all the branches.

git pull

You can run git merge into your current branch

git merge origin/<branchname>

once you have the branch in question in your repository as, say, anotherdev/master remote branch, you do the git merge anotherdev/master.