The fiscal sponsor must be a 501(c)(3) organization as recognized by the IRS. The fiscal sponsor cannot be an organization whose tax-exempt status is pending. The sponsored project must be able to qualify for exemption under section 501(c)(3), even if the project has no intention of ever applying for it.
Pressing the Ignore button on the listed invitation on the LinkedIn site is effectively a soft rejection. The message then moves to your Archived folder. You can later choose to open the message again and accept the request.
When dealing with an invitation you have a third button that you can press marked Report Spam.. This acts as a complete rejection and means the invitation disappears rather than moving to your archive box. You should use this option if you believe the person is not using LinkedIn for legitimate networking purposes.