Since the cost of a wedding rises with each guest, the best way to cut costs is to cut the guest list. There are some simple ways to decide who gets invited with a guest and who doesn’t.
If someone is married both the husband and wife are invited, even if you don’t like the spouse. I was once asked by a bride if she had to invite her friend’s husband because the she didn’t like him. The answer is yes. You always have to invite the spouse.
If a couple is living together, because they are dating—not just roommates— then you invite both of them and you put both of their names on the invitation.
You can decide your own “rule” regarding friends who are dating someone. Many couples use the six-month rule: If a couple has been dating (not living together) for six months or more they both get invited to the wedding. But you can decide the length of time—four months, 12 months.
If a group of guests are all single then they don’t need to be invited with guests, however if the majority of a group is all attached except for one or two people you may want to invite those singles with a date so they don’t feel like the odd ones at the table.
Mind you, these “rules” are just suggestions (remember Capt. Barbosa: They're guidelines) and you should use your own judgment. Only you know how your family and friends will react if they are or are not invited with a guest.