You make your own center.
If there are 5 others in your group, each member adds a border to your center.
Usually the widths of each border is predetermined.
You have a certain amount of time to add the border on, and then send it on to the next member.
There is a list that you follow, of which quilt top you will get, and who to send it to next.
The amount of time this takes, depends on how many are in your group, and the amount of time decided that you have on each border.