I was struggling with the same thing with my guests. My FIs family mostly lives in England, and I've never met them so it was difficult to navigate.
For those that are married, my FMIL actually filled me in - I sent her the list of who we had and when she sent back the addresses she included the names of the married couples. She also let me know the best way to send along the invites to the second cousins (my FI is the same age as them and grew up with them for a bit) was to send it to the first cousins that are their parents. Which helps me - because now I can shove a bunch in a bigger envelope!
When it came to anyone who I didn't know the status (and because we are so far out I didn't want to put the name of a bf/gf just in case something happens within 9 months), I addressed it to the primary invite, and let them know that they could expect an invitation for them and a guest shortly. That way they can pick who they want and aren't obligated to ask a bf/gf right now (where as if the name was on the envelope - awkward!).