When this happened to me while planning a trip with my DH, DD, DS and my parents; my dad's account would not play nice even though I had invited him. It turned out that Dad had more than one account and his acceptance of my invitation and the account with his ticket were not the same. This took a call to the
MDE tech folks because they had to combine Dad's accounts. ...to the "other" account where he had not accepted my invite, but MDE tech set up the invite to the now good account and accepted for me. (Since it had been done by Dad on the now useless account and Dad had given me his account information, they knew I wasn't trying to hijack Dad's accounts without his permission.)
It took about 30 minutes on the phone, but got worked our very courteously for us. Good luck to you