1) Go to 'Co-workers & Groups' link under 'Admin' on the left navigation

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2) Click on the group name on the right. You will see all the agents present in the group in order

3) You can now re-arrange the order of the agents by dragging and dropping them within the group.

Eg: Lets say you have 3 agents in the order  'A', 'B' and 'C'.  You can now drag agent 'C' and place it above agent 'A' to change the order to 'C', 'A', 'B'. 

After re-arranging, the calls to this group will first got to agent 'C'. If 'C' is busy or OFF then it  will go to 'A' and so on.