How do I customize messages people get when they register for a group or event?

To edit the default email messages sent from an event/group registration:

Go to Groups & Events > Settings > Browse Groups Page.  Scroll to the last section labeled "Other options." 

The default is set to "Allow self-registration for groups/events."  Having this option checked will open up the text boxes to customize the default messages to both the registrant and the leader of the group/event. Make sure this is the case for your organization and click save.

Go to the group and click Registration > Settings, scroll down and find the box labeled "Registrant Confirmation Email."

From here, you can add a Custom Registrant Email Subject line and create a customized message for the registration with any additional details (ex: packing list, what to expect, etc..). You can also customize a message to the leader by editing the Leader notification email box. Once you have finished customizing, click Save.

Note: if you copy a group (Ex: go to the group and click the blue Edit > Copy Group), this will copy the Registration Settings, including the customized messages you've done for one group/event, to the other.