First of all I want to say how kind and thoughtful it is that you want to be supportive of friends who have GAD. I wish more people were like you!
I think midnight's point about being understanding if/when people need to cancel going somewhere due to their anxiety. I've been known to cancel at the last minute - I hate myself for doing it but sometimes there are things I just can't face.
Having said that, when I want to try something or go somewhere new, I give myself a rule that I can leave as soon as I want. Even if it means walking through the door, staying for 5 seconds and then going straight home again.
Sometimes it's the worry that I won't survive the whole of an event that puts me off going, if that makes sense? So if I feel I have to stay at a friends for a party for several hours i'm more likely to cancel. But if I know I could go after 5 minutes, i'm more likely to try.
I'm sure you don't give out any cliches but saying things like "Don't worry!" just isn't helpful.
If a friend has shared something they are worried about I think it's better to give evidence as to why that worry/anxiety is unlikely to happen is far better than saying that it's not going to happen. Reasons are helpful.