Well, it would be a nice gesture given your venue capacity and budget allow for it, but do not feel obligated to! Imagine inviting a handful of high school friends and their plus ones, then getting stuck with an extra 40 plates to pay for? If you have a big family and a tight budget, you certainly cannot be adding that. However, there are a few exceptions. Allow close family members and the bridal party to bring dates, but ONLY if they have been dating for a decent amount of time. Keeping the guest list minimal is very important to limit stress (AKA chasing down RSVPs) and staying within your budget. Everyone should be able to understand that.