A quick Google search uncovers the Chilkat C/C++ TAR Library. A sample of its use is available here: Chilkat C++ Examples: Create TAR Archive
Another possible option is libtar (note, however, that it is a C library).
And, of course, you could read the TAR file spec (all of what you need is linked through Wikipedia) and implement your own library. It looks like someone has already done that here, with the source available.
