Same issue here, guild roster shows no one is online even though my husband is online. Had to wait for him to bring his character to where I was so I could party up with him because he can’t invite people.
I have a feeling it’s a pretty global bug. Certainly nearly every one of my friends. BlizzardCS on Twitter acknowledged it about 3 hours ago, but nothing since.
Interestingly, it seems like a few of our friends are coming back to being available for invite. Maybe 9 of my hundred-ish are working.
im not sure when it got fixed but it seems to be working on my end now
Deleted my post because I am happy to report it’s working now and I can invite 
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