It's not uncommon for a workgroup to include team members that are outside the originating organization. This can happen when you're dealing with external content providers, clients, partners, consultants or sub-contractors. Oftentimes, these kind of extended team projects pop up quickly, and you need to find a reasonable place to share files and other project content: graphics, documents, links, bios, schedules, etc.
Most internal systems present some difficult hurdles in these cases. If your organization is running SharePoint, it's difficult to authenticate people who aren't in the company directory. You can ask your IT department to add external people to the directory for the sake of the project but the request, if it's granted, might take weeks to be delivered.