There are quite a few coops in Chicago, and most require that all adult occupants (excluding live-in help) are approved by the board prior to your purchase. Renting is often expressly prohibited except under extenuating circumstances, and only then with a complete application and board approval. Most coop buildings don't contemplate renting a portion of your home as in a roommate situation, but my experience tells me that they would object.

To learn more about the pros and cons of coop ownership, you might want to check-out this recent blog post:

There actually is no deed on a co-op. You own shares of a corporation. The corporation's board of directors makes the rules and they must be adhered to. You would need to check if the board allows you take someone in, because most co-ops must be owner-occupied. In fact when you purchase, the board always wants to meet anyone who will be living there.

check your charter/prospectus...each board is different. My personal prediction is that as long as they are not on the deed or renting from you, who you room with is your own business; but check your charter.