So, for example, after registration, user is automatically assigned to the Joomla usergroup "agent-basic", then I also want "agent-basic" populated to the cb_usertype field.
You need 2 actions for this. Please see the below examples.
Type: Usergroup
Trigger: onAfterUserRegistration
Access: Everybody
Mode: Add Usergroup
Groups: agent-basic
Type: Field
Trigger: onAfterUserRegistration
Access: agent-basic
Field: cb_usertype
Operator: Set
Value: agent-basic
You don't really need the first action though if you intend on making everyone basic at time of registration as you can set the registration usergroup within Users > User Manager > Options. If you set everyone to basic by default then all you need is the below.
Type: Field
Trigger: onAfterUserRegistration
Access: Everybody
Field: cb_usertype
Operator: Set
Value: agent-basic