I think people feel more entitled to hang out at a chain than at an independent shop. All the suggestions here are pretty good, so I would just say ditto. However, when I approach the table, I politely ask "Hey Guys, what can I get you today?" as in it's not an option. This seems to work better than the more passive "Can I get you something?" but again, be very polite and upbeat. 9 out of 10 times that works.
...I bring out the secret weapon. That crazy customer that's always there and can talk and talk and talk and talk. I do a quick chat with the freeloader, get there name and then say, "hey ____, have you met loco before?" Then let the customer do my dirty work =P
Not run across that too much. Most times a group will take 5-10 minutes to assemble and come up and buy something after introductions. More often they'll have a laptop and come in to use the wifi without buying anything, in which case we boot them off of the network. When they ask about it, I tell them that they need the access code printed at the bottom of their receipt. Most will then begrudgingly buy a small coffee, which they then charge on their @#$% Visa Rewards card. Thanks.
A stop at the table with a tactful "can I get you something", repeated every 10 minutes or so, should suffice. They are welcome to feel insulted if, on your second or third visit, you inform them that your tables are for customers. If you've approach the situation reasonably, a reasonable person should not have any problem. Its not "customer service" unless you are dealing with a customer.
Obvious exceptions if you have good paying customers that have nowhere to sit. In which case, your responsibility is with your customer. Kick the freeloaders out!