In webmail click on the icon for your address book:

Click on the + sign to add a new group
(at bottom of page and graphic)

Create a name for the group

Press ENTER when finished typing the group name.

Select the group so that it is highlighted

Add a new contact by pressing the Add Contact button

Fill out the contact information as completely as needed
Only the email address and display name are required.

Contacts added to the group will show up on the right in the contact column

When finished adding contact return to the wegmail screen by clicking on the Roundcube icon.

To use the distrbution list, open a new email and tye the distribution list name into the to, cc or bcc field of the email

Be sure to press ENTER when you are finished typing in the name and it will be replaced by the emails on that distribution list

You may edit the list or add more addresses.