class SomeReactComponent extends React.Component {
    constructor(props) {
        super(props);
        ...
    }

    ...

    render() {
        return (
            <span>
                <p>Text that should be displayed to anyone!</p>
                { Meteor.userId() ? (
                    <button id="btn">
                        This button is only available to logged in users
                    </button>
                ) : ""}
            </span>
        );
    }
}

import React from "react";
import {createContainer} from "meteor/react-meteor-data";

class SomeReactComponent extends React.Component {
    constructor(props) {
        super(props);

    }



    render() {
        return (
            <span>
                <p>Text that should be displayed to anyone!</p>
                { this.props.authenticated ? (
                    <button id="btn">
                        This button is only available to logged in users
                    </button>
                ) : ""}
            </span>
        );
    }
}
export default createContainer(()=>{
  return {
    authenticated: Meteor.userId(),
  }
},SomeReactComponent);