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

Starting a new topic

If you are new to Groups.io, you probably received a welcome message with the group home page URL and the email address for the group.

How to send a message through your group homepage

Advantages:

Groups.io autosaves draft posts.

Easier if you want to use a hashtag.

A hashtag starts with a '#' and doesn't contain any spaces. It can be used to quickly locate/group related messages, manage topic duration, reply-to settings, and customize many other aspects of message delivery on a topic-by-topic basis

• Login to your group - here's how:  https://groups.io/g/GroupManagersForum/wiki/Logging-In
• Select New Topic from the menu on the left.
• Put in a good subject (not Help!). This means people are more likely to read your post and it is easier to find it again using the search function.
• Decide if you want to use a hashtag or two (Some groups don't allow them; some groups require them.) If available, use the dropdown menu to pick.
• Type your post. You may be able to change font, size, etc.
• Click on Send.

How to send a message by email

Advantages:

Easy to do. Just use the email account that is registered with Groups.io

You can set up an alternate email address for sending if you wish (an email alias). You can only send from there, not receive email.

https://groups.io/g/GroupManagersForum/wiki/Profile-and-Email-Address-and-Merging-Accounts%2C-Subscription

• Go to your email on your device (computer, phone, etc.)
• Just email as usual.
• Use a good subject line. See above.
• Be sure to send from your correct email address - the account registered with Groups.io.
• If you use Gmail, see this page about getting a copy of your own sent emails.
  https://groups.io/g/GroupManagersForum/wiki/Gmail