Our circle is very similar. If it's a party, my conversations generally go like this:
Me: "Oh, you don't have to bring anything! We're just looking forward to seeing you!"
Guest: "Really, it's no trouble, and I'd love to help out."
Frequently, Guest will suggest something here. "I could bring a cheesecake/salad/bottle of wine." If they say that, I say, "That sounds great!" If guest doesn't actually suggest something:
Me: "It really isn't necessary, but I'm serving X. Maybe an appetizer/side/dessert/booze?"
If you're having a lot of people, be careful not to tell everyone the same thing
If it's a smaller dinner, with just us and another couple, I'll usually give them the menu. That generally works. But if they're very insistant, then I'll suggest they bring something to drink.
If people are asking you what they can bring, they won't (well, shouldn't!) feel like you're insisting on something when you take them up on it!