We are 4 traveling by United on coach award ticket. This is total 4 leg journey (including return) within US. For couple of segments I am unable to select seats for 4 of us, since there is only one seat available. But it allowed to purchase the ticket and displayed message saying - unable to assign seats, and same will be allotted during check-in. Now my question is, if that particular flight is overbooked, shall we kicked out? or rebooked for next flight etc...?? How can I make sure the seat assignment for my flight? Thanks.