+ <channel> does not have to exist, but if it does, only members of the
+ channel are allowed to invite other users. If the channel mode "+i"
+ is set, only channel "half-ops" (and above) may invite other clients,
+ and if channel mode "+V" is set, nobody can invite other users.
+
+ References:
+ - RFC 2812, 3.2.7 "Invite message"