Troopr automatically sends reminders 15 min prior to the end of wait time for the Standup.
If you have a Standup 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 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:
Login to https://app.troopr.io
Select Standups from the main menu and then navigate to your Standup
In the Reports tab, select the participants and send reminders (Make sure you are in the correct date).
Hit the "Send Reminders button as shown here