Yes, many times whilst on corporate rates. IHG really does encourage people to use their corporate code for leisure travel, and I suspect no-one they enter into a corporate contract with (worth many, many 1000's of nights to them) would stand for a charging structure that introduced multiple people charges.

Even when there are no extra charges, I've very often found that my corporate rate - particularly if it's individually negotiated as opposed to the generic discount - is the cheapest rate.