Once you have a check-in configured, everyone you've selected to fill it out will receive a reminder each day it's due.
Note: If the Check-in allows individual team members to customize their reminder times and if the follow up time is scheduled to occur before then the follow up is re-scheduled for 1 hour after the reminder time.
By default, each person will receive a reminder by email:
And if you have Slack connected, each person will receive a DM on Slack (note that the Jell Slack bot doesn't support check-ins at the moment):
Each user can configure where they receive reminders by going to Notifications from the menu:
From here, they can toggle if they get notifications from email or Slack: