You'll first need to click on the "Override Default Profile" button to be able to change the settings at the Trip Group level. You'll now be able to select the Send Traveler Alerts and Send Opt-In Emails check boxes, then click Save. Please see the below screenshot for reference.
Please note: Clicking on the "Override Default Profile" button overrides your default account settings. If you have Trip Assist enabled at the account level, please ensure it is enabled at the Trip Group level after you've clicked on the Override button.