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.
These are precisely the sort of questionable connections that can ruin a person's LinkedIn experience, and I'm not prepared to take the risk. Also, I noticed something of a red flag when I examined your invitation, and I feel it needs to be addressed. You and I are former acquaintances, if that.