The first important obligation a guest has upon receiving a wedding invitation is to respond immediately. If your invited friends and family didn't complete this simple task, you'll need to get on the telephone (or to solicit the help of your mom, mother-in-law to-be, or bridesmaids) and politely ask these guests if they plan to attend.
As for your friends who are expecting to bring uninvited guests, it is impolite of a guest to ask, or even worse, assume that he or she can bring a date. You're not rude in saying no. The reality is that weddings are expensive; they usually can't be dating events. Who is traditionally included? Partners of invited guests should be included in a wedding invitation, whether they are married, engaged or living together, and whether or not anyone in the wedding party knows them. Allowing single guests who aren't attached to a significant other to bring a date is a thoughtful gesture but not required.