Send a message to all users (like a broadcast)

adam deltinger 

its not currently built into Teams as has been discussed in this thread.

However you can create this with not too much effort using PowerAutomate and optionally a PowerApps as an input form without a need for additional licenses.

the flow can 

1. get the members of any group in Azure AD 
2. filter the list to get rid of non-enabled users and guest users 
3. Apply to each, for this  use Settings > Concurrency = On ,
   Set to 20 .. 50 to send messages in parallel
4. Post a message as the Flow bot to a user 
   here you can set isAlert=true, and a Summary with the alert that is shown in the toast

By using the parallel / concurrent option , the time needed to send individual messages to a many different users is reduced by a factor of 20..50.  each msg post seems to take in the order of 1 second.
Estimated throughput : 
1000 users / 50 = 20 seconds  or about 3.000 users / minute 

Note that while this works well up to a couple of thousand users, I do not think you can just scale this to 100.000 users as other other restrictions will apply , such as the below.
for that scenario some additional logic will be required 

Limits 

https://docs.microsoft.com/en-us/power-automate/limits-and-config

https://docs.microsoft.com/en-us/graph/throttling

and in the licensing guide : 
https://docs.microsoft.com/en-us/power-platform/admin/powerapps-flow-licensing-faq

you can build on this concept by adding logging of success / failure to notify, and ask for acknowledgement and store that in a log (Excel / SPO List) or database of some kind.

see attached for a quick Proof of Concept