As an added suggestion, perhaps add an optional timer?
So:
/ch f add <username> 10:00
"<username> has been added to faction chat."
ten mins go by
"<username> has been removed from faction chat."
It would add the user to the channel for 10 mins, then auto remove them. That way it'd be possible to buy time in a faction's chat, or if you were intent on someone only having x amount of time to explain something, good for quick interviews or demanding an explanation for violence.
A similar suggestion would then be Party Chat. It'd be another channel that only people the channel opener invite will be in. The OPs suggestion would be needed to make this one happen, and my timer suggestion would fit nicely as well.
Though having tons of extra channels popping into and out of existence might strain the server, idk.
A final added suggestion would be to add an optional price (faction decides its amount) so that using it costs silver. It'd make it a reasonable way to keep the command public in the faction instead of officer based. And it'd also be a mild silver sink. Then usage would become:
/ch f add <username> [timer]
"[Cost] silver has been deducted from your account and <username> has been added to faction chat."
I guess it'd be another option to make the payment eat the silver entirely, or give it to the faction (first is a better silver dump, second is a better form of income for factions).