Groups are a new feature available within professional and enterprise plans - as used to facilitate configurable access control (See "What is configurable access control?").
See "How do I upgrade from a free plan?" if you need to upgrade.
Account architects can add/manage members within a group.
To add/manage members within a group, as an architect, click first on the "Members > Manage groups" option in the top menu.
Next, click on the group you wish to add/manage members within.
In the group window, ensuring you are in "Edit" mode (blue pencil action button selected), you can add account members to the group individually via the "Add members" dropdown (using the search field to delimit the dropdown options).
Previously added members can be removed from the group by clicking on the "Remove" link beneath the member's avatar.
Changes are automatically saved.