Troopr automatically sends reminders 15 min prior to the end of wait time for the Check-in. For example, if you have a Standup Check-in scheduled for 9 am with 30min wait time, the first notification will go out to participants at 9 am and a reminder will go out at 9:15 am. If the Standup Check-in is cross timezone one, all participants will receive reminders at the same time irrespective of their local timezones.
In case you want to send a manual reminder with an answer button to select participants, you can do so in the web app by following these steps:
Hit the "Send Reminder" button as shown here.