You can make a do command like I did, or you could get on the bot's partyline and do .dump join #channel <password if any> ofcourse you would have to .+chan #channel on the bot's console for it to stay in the channel, for it to add it as a dynamic channel.
If there is no bind available according to tcl-commands.doc, then you can always use a RAW binding. In this case with "INVITE" as the keyword.
Note that it seems not necessary to do this. If the bot has the channel in its channel list but can not join it because the channel is invite only, and the bot then receives an invite, it will automatically join the channel.