Friends can say it up front "Ce soir, c’est nous qui vous invitons" (Tonight, it’s on us.). Or, just grab the bill when it arrives. Friends often argue on who will pay the bill, meaning everybody argues that they want to pay the whole bill. And it is often settled by "Non, j'insiste. Vous nous inviterez la prochaine fois" (No, I insist. You'll invite us next time), or "Non, non, c’est à notre tour, c’est vous qui nous avez invités la dernière fois" (No, no, it's our turn, you invited us last time.)
Be aware of the difference between, "Je voudrais vous inviter au restaurant" (I’d like to invite you to a restaurant.), which means the person will pay the whole bill, and "Vous voulez venir avec nous au restaurant ce soir?" (I would you like to join us in a restaurant tonight?). Or, more casual: "On se fait un resto ce soir?" (Why don’t we eat out tonight?), which is unclear on who is paying.
For more info on this subject, check out my French Table and Hosting Etiquette Cultural Guide
