'Country club casual' is a fancy way of saying business casual; recommended attire for men includes a knit polo shirt with khakis, while appropriate attire for women includes a sun dress with flats. However, when an outfit is in question, it is best to err on the side of dressy.
Many country clubs are particular about the dress code for its members, and usually have a printed dress code. New members receive a copy of the dress code at the time they join, and in some cases, clubs send the dress code in monthly newsletters, as a reminder. If members plan to bring guests, country clubs expect those guests to follow the same rules, so it is up to members to explain them to the guests, or provide them with a printed copy of the attire requirements.