Invite

If a user has ops in a channel they may invite other users to it. To issue an invite use:

/invite nick #channel

The user, nick, will then receive a notice along the lines of:

nick has been invited to #channel

Being invited to a channel means that a user may join the channel even if they are banned, or if the channel has modes +i, +k or +l set.

Some clients will automatically join a channel if they're invited to it. This can be handy but if abused can result in unfriendly attention from the opers.