The other answers here are generally right - I just thought i'd add that you're probably not going to get a real chance to do anything with the clients. After all, the CEO is going to be trying to sell them something.
I know he said you'd all have a chat, but typically the sell-side goes overlong, and then the chat gets cut back.
So in the office, it is unlikely they will talk to you much more than "hello".
If you are brought into any meetings, as @ChristopherEstep & @ChrisLively have noted, your absolute goal in the meetings is to not rock the boat. If you have any good questions or suggestions, then I would encourage you to bring them up during whatever meeting you're in, as long as they're pertinent to the meeting at hand.
Now, this might change depending on the involvement you have with these guys - when you say you are heavily involved, are you client facing? ie are you chatting to them determining requirements? Or are you given the requirements by someone else in the company?
If you're client-facing then some member of the client party may know who you are, in which case...
You might get invited to some sort of after-hours drinks thing. The important thing here is to
- not assume familiarity with the clients and get v. drunk
- don't get v. drunk
- just don't. Also, don't mention any issues you might have with your company, or disagreements you've had with anyone about anything in the company. You're trying to make your company look good, not yourself here (because it is unlikely you will get too much gain from it).
- Also, keep politics out of it too. Pretty much, just have a nice time with them talking to them about stuff that might be interesting. And don't get v. drunk.