The easiest way is to:

1. Install the bot for the team
2. Query the Team Roster — The link in Step 3 has an alternative way to do this towards the bottom
3. Create a conversation with the user and send a proactive message

There’s a lot of code in those links and it’s better to just visit them than to copy/paste it here.

The end of Step 3 also mentions trustServiceUrl, which you may find handy if you run into permissions/auth issues when trying to send a proactive message.

Edit for Node:

Install Necessary Packages

npm i -S npm install botbuilder-teams@4.0.0-beta1 botframework-connector

Note: The @<version> is important!

Prepare the Adapter

In index.js

const teams = require('botbuilder-teams');

adapter.use(new teams.TeamsMiddleware());

Get the Roster

// Get Team Roster
const credentials = new MicrosoftAppCredentials(process.env.MicrosoftAppId, process.env.MicrosoftAppPassword);
const connector = new ConnectorClient(credentials, { baseUri: context.activity.serviceUrl });
const roster = await connector.conversations.getConversationMembers(context.activity.conversation.id);

Send the Proactive Message

const { TeamsContext } = require('botbuilder-teams');

// Send Proactive Message
const teamsCtx = TeamsContext.from(context);
const parameters = {
    members: [
        roster[0] // Replace with appropriate user
    ],
    channelData: {
        tenant: {
            id: teamsCtx.tenant.id
        }
    }
};
const conversationResource = await connector.conversations.createConversation(parameters);
const message = MessageFactory.text('This is a proactive message');
await connector.conversations.sendToConversation(conversationResource.id, message);

Trust the ServiceUrl, as Necessary

Read about it. You’d want this before the message is sent.

MicrosoftAppCredentials.trustServiceUrl(context.activity.serviceUrl);