I have had this problem when checking in my 5 family members (on one PNR) online. You can get around this by splitting up your check-in. Just check-in 2 or 3 or 4 passengers first, select their seats, then check-in the rest. You can check if there are 5 seats that you like when you get access to the seat map, and if you don't like what you see, cancel the check-in and keep the seats you already have.