We get this question quite a bit so we've posted a 'Best Practices' page explaining how both IntelliGantt and the IntelliGantt Add In for Microsoft Project work with SharePoint members. They also work with Outlook contacts and resources you add manually, but the SharePoint case is the most interesting.
The two use cases this page covers:
1) Building a simple resource pool from a SharePoint site
2) Turning resources that are really roles into actual SharePoint members (and yet they're still project members-- it's magic!)
Well, not quite magic. Basically, it all comes down to the email address. In MS Project, resources support an Email Address field. This is not in the default view so you have to insert the field into the 'Resource Sheet' view. If the Email Address field for an MS Project resource matches the Work Email field of a SharePoint member, they're connected! That's the manual approach.
Because connecting resources to SharePoint members requires this bit of housekeeping, there's really a third reason for the 'Best Practices' page-- how to add 'SharePoint Approved' members as resources to your project. With the IntelliGantt People pool, once they've been retrieved from SharePoint its as simple as clicking a checkbox.
Hopefully the 'Best Practice' page is more than a good read-- we want it to be helpful too!