Splash shows Salesforce-related fields right in your Event Level Guest List. As long as you have a Salesforce Integration set up, there are three columns you can choose from:

• Salesforce: Sales Cloud Lead / Contact ID
• Salesforce: Sales Cloud Campaign Member ID
• Salesforce: Sales Cloud Person Object Type

Including these fields to your ELGL will also allow you to export this these fields to a .csv. If you have any questions on exporting your guest list, here's a great article to get started: How do I export my guest list?