My JIRA instance needs to include not only folks from within my organization but external contractors as well. The challenge I had was how to restrict access for the external folks to only a particular project.

It's taken a bit of fiddling to get this figured out as it's not entirely clear in the Atlassian documentation. I think JIRA is an amazing product but there are times when I find the configuration of the system to be quite baroque.

Following these steps should allow you to accomplish the restriction goal and still have time for lunch.

Create a contractors group.
Add the contractors as members of the contractors group and be sure to remove them from the users group.
Assign the contractors group to the users and / or developers project role in the target project.
Assign the JIRA Users global permission to the contractors group. This assignment is needed since they are not members of the users group.

That should do it. Do let me know if you find a more elegant way than what I've presented above.