Basically the way we were initially doing the kara groups were that the starting group that had been in kara from when we started and was always there to run it were placed in group 1, and the newer people were placed in group 2. (this way group 1 still had its best chance of getting as far as possible in kara).
When group 1 killed prince we decided to split the groups up a little more so that the experience from group 1 would be able to help group 2 progress further through kara, and so that they could get the leadership that they were missing. As a result 3 of us (myself, narn and viper) swapped into group 2, and 3 of group 2 swapped up into group 1. This way group 1 still had the majority of its normal raiding group, and would still be able to maintain how it was going, whilst also letting the 3 new people in it get more experience, and it allowed the 3 of us that swapped to group 2 to lead the remainder of group 2 and to get them working better together.
On the day kara got reset, we got together the 20 people that were about to run (10 from group 1 and 10 from group 2 (it was the 10 that were on at the time, i'm guessing you were either a little late or whoever was inviting for group 2 initially must've missed you or something) and we swapped the groups around slighty so that the 3 swapping from group 1 would get replaced by people of the same class. This then gave us the 2 core groups for the week. Due to there being a lot more than 20 people that can run kara, all the other people currently keyed, that weren't in a core group were placed in a waiting list for the week, which means that if someone from either group doesn't show up, the people in the waiting list can replace them.
When kara resets again, we'll go through the same process again, letting some new players go through into group 1, and swapping around people who missed out last time with some that got in this time to let everyone have a go. The people that are there waiting everyweek however, and that are more reliable etc, will be given the go-ahead over those that only turn up once a week etc.
This is what we're aiming to do with the groups, it does mean that people will miss out occasionally (which is unavoidable due to our numbers) but we are going to try and ensure that everyone gets a go, if they are commited and do show up.