There are several different factions plugins to choose from so the commands may not work. First try "/f leave" if it is an unrecognized command do "/f help" if it is still unrecognized do "/help" and try and find what the factions help page is. Type in the commands if it offers one to find out how to leave. If you don't have permission to use "/help" then try asking around or private messaging people on the server to find out how to leave your faction. Your faction leader could always just kick you if you can't manually figure out how to leave.
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You use /f leave [faction name]
To answer a faction invite just do /f join <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>
someone who moderates the factions