You can't because you or you sitebot, like any other user, need to be invited to be able to enter an invite-only channel.
Your bot should be linked to your botnet so all the bots invite themselved when they need it.
You can also use network specific features. Some EFNet servers support the +I mode, some other networks have chanserv, etc.
Code:
∙φ∙ cmode +I - Invite Exempt. Allows a user to join a +i channel without an
∙φ∙ cmode invite, based on a nick!user@host match.
∙φ∙ cmode PARAMS: /mode #channel +I nick!user@host
Then, your users (and you) can use SITE INVITE <nick> so the sitebot invites them in the channel. People with partyline access can also chat the bot and ask him to invite them : .invite moreniu #yourchannel