Facing the same problem and I knew there would be an existing thread.

Based on my experience I would avoid Group reservations like the plague.

• Had to call Premier desk to add MP #
• Premier desk was unable to change seat assignment - transferred to "Group Desk" with long hold time
• Individual traveler is unable to do online or app checkin (despite having added FF #) - the whole group must do airport checkin. Ridiculous not to mention inefficient for UA.

After speaking with Group line the kind CSR was able to re-assign passenger to an aisle seat.

But next time I will ask for the Travel Agent ASAP and request preferred seats which are normally available when bookings are made months in advance. What idiots book people into middle seats (I gather on the improper assumption that everyone in the group wants to sit together).