invite <nickname> <channel>

You may invite another irc user to join a channel by sending them an invitation with INVITE. If a channel is join-by-invitation-only (mode +i), you must be a channel operator to invite people there. Some servers also require that you be a member of a channel to invite others when the channel is not invite-only.

To invite foobar to channel #blah:

    /invite foobar #blah

To join the last channel you were INVITEd to:

    /join -invite