Either ask a member or ask the Administrator of the server.
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It depends on if the server you are playing on even has factions. If so, type /f create (factionnamehere) to make one or /f join (factionnamehere) to join
To answer a faction invite just do /f join <faction name>
I assume you're talking about a faction invite. the command is /f join <faction>
The ip to join a server.
Unless your server has a whitelist or a blacklist, anyone with the IP address of your server can join it