Do people with ADHD happen to have a low IQ usually?

Absolutely not. Iq has little, if anything, to do with adhd. If you are distracted or inattentive you may "look" like your IQ is "lower." but, this is only an illusion. The distraction of adhd causes you to perform poorly on common indicators of intelligence. "perform" is key. Your actual intelligence is not changed.
They are smart! Sometimes it is the intelligence that makes them appear to have adhd. Very smart kids can get bored in the classroom. They may become distracted due to lack of intellectual stimulation. They may not have adhd at all.
More Than Average. Having adhd does not commonly lead to low iq. However, this connection is more common than in the general population. I would say it is "frequent", but it is not unusual.