Agatha was wearing this dress, along with a shawl (more for effect than because she needed it as she would have taken it off shortly after arriving) along with the necklace Severus gave her.
She would have stayed with Severus and mostly danced with him though she would also have danced with anyone who asked her. She would have been polite and graceful when they spoke to the others though she would have taken a moment to go and flail (genteelly, elegantly and quietly) with Mireille (and Jacqueline) about the wedding. She would have been quite happy to find a quiet corner and would have likely amused Severus by making very quiet, straight-faced but rather amusingly biting comments about what others were wearing.