When someone asks you to his or her home in the USA, it's very clear who the guest is and who the
host is. But invitation to restaurants are usually different. In many cases, it is the inviter who pays. But in
some cases, each one pays his or her own bill(账单). For example, when your friend says "Let's go and
get a beer," or "Want a cup of coffee?", it may be more of advice than an invitation.
     Some people like taking their friends to a restaurant for dinner instead of having dinner at home. In this case, the host is expected to pay and the guest may leave the tip(小费). If you want to invite someone for a meal at a restaurant, tell him or her: "I'd like to take you to …".
     There are also phrases that sound like invitations, but in fact they are not. People may say things like "We'll have to get together sometime," or "You'll have to come over and visit us sometime." They are the
expressions of saying goodbye. So you could answer "Yes, that would be nice," or "I'd love to," or
something similar.