A system and method are disclosed for enabling a guest user who is not a registered user of an instant messaging system to communicate by instant messaging with a registered user of the system. An indication that the guest user desires to communicate with the registered user by instant messaging is received. Browser-executable computer code comprising computer instructions for providing at least limited instant messaging functionality to the guest user at the guest user's client computer system, without requiring the guest user to download and install client side instant messaging application software and without requiring the guest user to register as a user of the instant messaging system, is sent to the guest user's client computer system. An instant messaging session between the guest user and the registered user is facilitated.