In the list of "Business relations", click the options icon for the party to be handled.

Select "Group" to start adding that party to a group.

A pop-up window appears.

To add the party to a new group, start by entering a "Name".

Click "Create", to create a group with the entered name.

The name will be added to existing groups, and selected under "Select one of your existing groups...".

Click "Add to group" to add the party to the selected group.

The party will be added to the group.