It must have the factions plugin.
The command it /f create name
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If it has factions installed, you type this in: /f create name Replace "name" with what you want your faction to be called.
To answer a faction invite just do /f join <faction name>
You use /f leave [faction name]
If you are in a faction, and want to teleport to your faction home, just type /f home.
I assume you're talking about a faction invite. the command is /f join <faction>