Asana is only letting me add 13 people to my workspace instead of the 15 it says should be included with Basic. For some projects I can have 13 and for others I can only have 11. I am using the desktop app.
The limit applies to all people in your org/workspace, not in any one project, so I think that may explain it. If not, please try these troubleshooting steps, then please 1) report your problem by going to Report a Bug, clicking the New Topic button at the right, and following the instructions, or 2) contact support directly with these steps.