We have 11 people in our group calendar and when creating an event I can invite them all except for 1 person - her name comes up with a red box around it for me however, other people can invite her to events. I'm admin and problem only started today, I've added same employee to other events no problem before now. 

- I checked that she's in the group calendar, she is

- I checked that there were no conflicting events, there aren't

- I checked that I can't add her to any other events or dates