In Admin, use the menu to go to Settings.
In Settings, go to Member Notices and click on +New Notice.
Click on Notice Type - Welcome - to see the list of notices available.
Click to choose the notice you want to create or edit.
Fill in the information on the form.
As a special case, the Guidelines can be sent on join and/or monthly, in addition to appearing in the left column.
Click on Add Notice.
See also: https://groups.io/static/help#membernotices
If you want to send a message on a regular schedule you can use a calendar event for that.