Sending noticable messages to "private" group?

I got a group where I have people who I need to send a message to once every while and they need to recieve it as noticable as possible. We have been using Discord but sometime the messages does not create a notification and the message are not read in the timeframe that it has to be read.

What application, solution can I use to solve this issue?

These are people who want the get the message and not just random people who I am spamming.

by Delite via TBN