As urbansombrero99 mentioned, this isn't available in your private "My Projects" workspace. The option to invite people is only available in a shared workspace. You can create a new shared workspace by clicking on the "New Workspace" button on the logged-in page:
Once inside the newly-created shared workspace, click on "Manage Users > Invite People":
Enter the email address of the users you would like to invite to this shared workspace, and they should get an email notifying them that they've been invited to your workspace. They will need to log into their own Axure Share account to accept the invitation.
Hope this helps!