When following the current documentation microsoft provides, it will most likely spit you out at this page. Which is a guide on building bots, and has no direct instructions for actually retrieving a user's ID for sending, that does not require the user interacting first.
I'm building an incoming webhook to a channel, not a bot. So how can I get the ID's of the users I would like to mention?
Assuming you have the correct privileges (I personally do not), there is a "unified" API that Microsoft provides that can supply the information here.
Otherwise, if you open the web version of teams, you can find the user ID in the URL of the conversation you have with said people.
teams.microsoft.com/_#/conversations/19:{you}_{the person your talking to's ID}@unq.gbl.spaces?ctx=chat