See also the official documentation:
Help Center > Members Manual > Working with polls

You must log in to your group site to create a poll. You must have permission to create the poll - in Settings.

Polls are sent as messages to members with the hashtag #poll in the subject line. (If the hashtag doesn't exist, it will be created when you post the first poll.)

Members must login to the site to participate. The email has a link "vote now" that takes the user to the page (if logged in) or to the login page.

To create a poll on your group site: 

Log in. Click on New Poll - lefthand menu.

Add a title, question and answers - note you can add more reply options by clicking Add Answer.

Choose any or all additional options.

Post poll.

Voting in a poll:

To respond to a poll, the user must first log in.

Click on the radio buttons.

Vote!

You can change your vote too.

Examples:

https://groups.io/g/GroupManagersForum/message/6763

https://groups.io/g/GroupManagersForum/message/29385

To find the results of your poll:

Go to the post in which you created the poll.

On your group site, click on Messages in your group site. 
Click on Topics beside the search button. Choose Polls.

You could also select Hashtags from the left menu and choose #Polls to list them all.

Click to open the message with the poll. Depending on your settings, only owner/moderator or all members can see the results and you can see who responded. 

The simplest way to share results of a poll is to email them to your subscribers.

Making the poll a "Sticky" keeps it at the top of the messages for all to find more easily.

Closing a poll:

Click on the More "hamburger menu" at bottom right and select Edit Poll.

There will be a Close Poll button at the bottom of the resulting screen. 

Modifying or preferences for polls:

You can edit your preferences in Settings, Polls.

You can also modify the properties of the hashtag #poll by clicking on the Edit button - for example, duration or make it owner/moderator only.

Polls have a new option to never show results to members.

Problems:

Q: Our Membership guy has initiated a poll, but now wants just to chase up those people who have not replied, to get them to take the poll. Is there any way to do this, or can we only send a chaser to all members (which will hassle those who have already responded)?

A: Sorry, not at this time. (2018-12)
You can send a member the message number - Each message has an individual message number (upper right corner). Right-click on the link at upper right of the poll and selected "copy link address." (The text displayed by your browser may be different.)

Q: Rather than logging into groups.io and responding to my poll, many of my subscribers are instead replying via email to the poll notification. As a group Owner, is there any way that I can add these responses to the poll results?

A: No.