The correct word is it. → "The revised meeting agenda is attached and it needs to be emailed to all participants before the meeting."
"It" is the correct pronoun because it refers back to the singular noun "the revised meeting agenda." "This" is a demonstrative adjective/pronoun (not used as a subject pronoun here in place of a noun already named), and "those" is plural, which doesn't agree with the singular "agenda." Always match the pronoun to the number and nature of the noun it replaces.