This plugin lets you set up a list of channels you would like to auto-join when invited. When invited to a channel on the list (regardless of who invites you), the plugin will automatically do a join command for you.
/invitejoin [#channelname]
This adds a channel to the list. If you don't give a channel name, it shows the current list. If you use -#channelname it removes that channel from the list.