Once logged into web client click the setting gear
Search for distribution and click on the link for Distribution groups
This will bring up the settings for Distribution groups you belong to, and Own
Double Click the group you want to manage, this will bring up the settings for that group. You can select membership and add/remove members.
You may need to select the Default Global Address list, you can then search for the user you want to add, click the + by the name to add, you can add as many as you need and then click save.
