I assume you're talking about a faction invite.
the command is /f join <faction>
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To answer a faction invite just do /f join <faction name>
To invite someone to your faction in Minecraft, do /f invite <player name>. They then need to do /f join <faction name> to accept your invitation.
You use /f leave [faction name]
If you are in a faction, and want to teleport to your faction home, just type /f home.
someone who moderates the factions