The couple does need their blank invitations to attend the ceremony: https://www.finalfantasyxiv.com/eter...?lng=en&rgn=na

"When the scheduled time for your ceremony arrives, both you and your partner, along with any guests in attendance, may enter the venue by speaking with Etoinelle. Please be reminded that only those who possess an invitation may attend the ceremony. This requirement also extends to you and your partner, who must be in possession of your blank invitations."