Install Group Reminder to your server, configure it and start. Then your team will use it from mobile apps or Web Browser.
You can install it in the cloud or on premises.
How to install Group Reminder.
Mobile: use IPhone 5S or later, or Android 4.1 or later. Server: it depends on quantity of users and their activity.
If you have 5-10 users, you can use very simple server or VPS/VDS.