The on INVITE event triggers when a user invites you to a channel.
Format: on <level>:INVITE:<#[,#]>:<commands>
Example: on 1:INVITE:#mIRC:/join $chan
on 2:INVITE:#:/join $chan | /timer 1 3 /describe $chan appears in a puff of smoke!
This triggers when a user with access level 2 invites you to any channel.
Note: If you want to automatically join a channel when someone invites you, it is easier to turn on the Auto-Join option in the Options dialog.