Depends on the countries in question. Many nations share a visa waiver program. As an example, people visiting from England do not need a visa to visit the US. As such, while the invite would be useful it does not need to be notarized in any way.
Additionally, you can not simply notarize just anything. To be notarized the document must bear a signature and have a place for the notary to attest to the validity of the signature. The notary does not attest to the validity of the information in the document, only the validity of the signature on it.
Depends on the countries in question. Many nations share a visa waiver program. As an example, people visiting from England do not need a visa to visit the US. As such, while the invite would be useful it does not need to be notarized in any way.
Additionally, you can not simply notarize just anything. To be notarized the document must bear a signature and have a place for the notary to attest to the validity of the signature. The notary does not attest to the validity of the information in the document, only the validity of the signature on it.