As you round up addresses (and spellings of names) for your guest list, you're going to want to find out: Are the single cousins dating anybody? What about friends from college -- are they involved? That's data you'll need as you ponder the questions, "Should I invite boyfriends and girlfriends?" and "Can I afford for everyone to bring a date?"
But ask carefully -- you don't want to create an expectation in their minds that you're inviting casual sweethearts, just in case it turns out you have to tighten your numbers. Try stealth tactics first: Check Facebook, and ask family members or mutual friends. Still not sure? Ask their mothers, or ask them directly. (Hopefully, you'd already know if they're engaged, married, or living with someone, but not always.)
Another tip: Build this database early -- then it won't be so obvious that you're asking for the invitations.
What's your stance on guests bringing dates?