I have the following working:
import com.atlassian.jira.component.ComponentAccessor
def customFieldManager = ComponentAccessor.getCustomFieldManager()
def groupManager = ComponentAccessor.getGroupManager()
def singleGroupCf = customFieldManager.getCustomFieldObjectByName("Department")
def group = groupManager.getGroup("Services")
issue.setCustomFieldValue(singleGroupCf, [group])
How can I set the group, based on a currently selected checkbox field called "Assign Department"