Post # 16
If you know her name, I’d put her name. Are you getting extra envelopes with your order? I’d have a few blank ones JIC. If they break up, just re-do them, Groomsmen + guest. But until then, I think its more courteous to use her name.
Post # 17
Are you inviting her specifically or are you fine with him bringing anyone? i.e. if the girlfriend is out of town for a work conference that date, are you just fine with him bringing his drinking buddy to hang out with at the wedding? And guest implies you are fine with them bringing anyone they choose to accompany them regardless of gender or relationship.
Personally, if you know they are dating someone exclusively, regardless of their living circumstances, you should put the person’s actual name down. They are a social unit.