If you would like to add an agent to a group you may do so using the Atomicorp GUI or using the terminal.
Adding and Agent to a Group in the GUI
In your Atomicorp UI, go to Atomic Protector > Agent Management
Click on Agents in 'default' group, select the agent you want to move and then in the upper right select 'assign selected' and click apply
In the next window, choose the agent group to move to and select 'save'
Adding an Agent to a Group in a Linux Terminal
As root run
/var/ossec/bin/agent_groups -a -i <agent_number> -g <group_name>
You will be prompted y/N
After selecting 'y' your changes will be confirmed