If I buy 1 share of a company's stock, do I get to attend and vote at the next shareholder meeting?

You always have the right to vote your shares for the annual general meeting (AGM).

Whether you can attend the AGM in person is up to the individual company. Some will let you attend with a single share. Most will set some minimum number of shares you have to own to be allowed to attend in person (or else restrict attendees in some other way. You can only fit so many people in one ballroom).

The first one I found via google was this from Telefonica where they say you need to own at least 300 shares (~€1,000 worth) to attend the AGM in person.

A company will have specific rules for attending meetings. Since you ask about MSFT, I found this (2015) example of their rules, which will be updated regularly to account for pandemics and such: https://www.sec.gov/Archives/edgar/data/789019/000120677415003229/microsoft_def14a.htm#ProofofOwnershipRequiredforAttendingMeetinginPerson

In order to be admitted to the Annual Meeting, you must present proof of ownership of Microsoft stock on the record date. This can be:

● a brokerage statement or letter from a bank or broker indicating ownership on October 2, 2015,

● the Notice of Internet Availability of Proxy Materials,

● a printout of the proxy distribution email (if you received your materials electronically),

● a proxy card,

● a voting instruction form, or

● a legal proxy provided by your broker, bank or nominee.

Shareholders and proxy holders must also present a form of photo identification such as a driver’s license.

Shareholder activism

With one share you may have the right to vote. With one vote per share, that won't bring you very far.

More importantly: with one share (or a larger minimum, depending on policy) you may have the right to attend and speak at the meeting. You can try to convince others of issues you find important (of course they should be related to the business of the company involved). Exactly this strategy is followed by shareholder activists, which may be political activist groups for social or environmental change:

Some of the issues addressed by shareholder activists are for social change, requiring divestment from politically sensitive parts of the world, for example, greater support of workers' rights (sweatshops) and/or more accountability for environmental degradation.

Here is an article by Greenpeace calling on people to use shareholder activism to hold corporations to account:

And if you do have shares in fossil fuel companies, use your voice as a shareholder and be an activist in the boardroom! Greenpeace is working with shareholder activists all over the world to turn their voices into megaphones.

This is just an example; issues raised by shareholder activists can be many. You may need more than one share, but for larger political groups, investing €1000 (or whatever the minimum is) in Shell or Exxon in order to get the right to speak at their shareholder meeting is an entirely realistic prospect.

Of course, whether such shareholder activism is actually effective is a different question altogether, and beyond the scope of this question (and probably off-topic on Money SE).

It depends on which stock you buy. Per your example, if you bought 1 share of MSFT prior to the record date, you would be entitled to one vote per nominee/proposal:

Shareholders as of the record date are entitled to vote. Each share of common stock of Microsoft Corporation (“Company”) is entitled to one vote for each director nominee and one vote for each of the proposals.

You would not have to attend the shareholder meeting in person. You could mail in the proxy ballot, vote by phone, or vote online.

If you bought a different company's stock, you might not be allowed to vote, especially if the company has different classes of stock. For example, Google (Alphabet) has 2 classes of stock that trade publically:

There are two ticker symbols for Alphabet Inc. on the NASDAQ stock exchange: GOOG and GOOGL.

The two tickers represent two different share classes: A (GOOGL) and C (GOOG).

There is, however, one crucial difference. A shares receive one vote, C shares receive no votes

So, if you want a vote at the shareholders meeting, buy the A shares.