When creating or managing your event, you can enable the 'Require verification to join' option. Once this option is enabled new participants can only join your event when they enter a valid verification code. By default every verification code can be entered only once. After that the code is marked as used and cannot be used anymore.
Without registration data
If you don't have a registration system you can chose to simply upload a list of codes which can be used by participants to join. This way you can still limit access to your event without having to use a registration system.
As a race organizer you can manage the list of valid verification codes. A typical setup is to create a unique complex code for every registrant to your event and email their personal code to the registrant with the link to your sodisp event. On your event setup page you can upload the list of valid codes (one code per line). Once a participant uses their personal code it is removed from the list to prevent them sharing their personal code with friends.
Optionally it is possible to mark verification codes as reusable (great for marketing campaigns) by simply adding a '+' to a verification code.
Mail merging personal activation/registration codes
See this article to invite participants to your challenge.