We added a new feature to HackerRank for Work: reminder emails. For each invite you send, you can opt to send one reminder email with customizable contents. This is a simple way to increase the number of candidates that attempt your tests.
Using the reminder emails feature is simple. Turn it on, select the timing of the reminder, and set the contents.
You can enable reminder emails for a test from the miscellaneous section of its advanced settings. You can also set a default value for all invites you send in your personal user settings.