You shouldn't need to wait for the leader to check in. You should be able to just go along and check in once the flight is open.
Online check in may not work as they will have probably assigned the seats to your group in one section and they won't want you moving out of that section and breaking the group up. However if you ask to change your seat at check in then they are unlikely to say no. Your FF card should be added then too. You may not get the number of miles/tier points you expect as if it's a group booking and has used one of the Group Booking ticket classes then it may be low as you will have probably paid less than 10 individuals who booked the flight at the same time as you, so FF rewards will possibly reflect that.