zulip/static/templates/compose_invite_users.hbs

<div class="compose_invite_user" data-user-id="{{user_id}}" data-stream-id="{{stream_id}}">
    {{#if can_subscribe_other_users}}
    <p>{{#tr}}<strong>{name}</strong> is not subscribed to this stream. They will not be notified unless you subscribe them.{{/tr}}</p>
    <div class="compose_invite_user_controls">
        <button class="btn btn-warning compose_invite_link" >{{t "Subscribe them" }}</button><button type="button" class="compose_invite_close close">&times;</button>
    </div>
    {{else}}
    <p>{{#tr}}<strong>{name}</strong> is not subscribed to this stream. They will not be notified if you mention them.{{/tr}}</p>
    {{/if}}
</div>