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.
Messaging a non-friend should be treated with caution. If you message non-friends too often, or too many people report your message as unsolicited or unwanted, your account is automatically flagged by Facebook. People who send too many unsolicited messages may have their accounts blocked or permanently disabled.