From your use of underscores, I assume you're using the Python BDK.

You can use the slash command decorator and include an @mention as an argument in your pattern.
https://symphony-bdk-python.finos.org/markdown/activity-api.html#slash-command-pattern-format

For example,

@activities.slash("/ping {@person}")
async def on_echo_mention(context: CommandContext):
    person = context.arguments.get_mention("person")
    name = person.user_display_name
    uid = person.user_id
    await bdk.messages().send_message(context.stream_id, f"Hello <mention uid=\"{uid}\" />")

There's no need to do any validation since an @mention will always resolve to an actual user. However, this could be either an internal or external user - so be wary of using the user id immediately to create a new IM and send a message if your bot isn't connected to the external user.