When you close a lead as 'Won' or 'Lost' you can elect to add the contact to one of your 'Contact Groups'.  This can be done at the time of closing your lead, but it is also possible to set via Settings>My Company (with the Company Admin role) or Settings>My Yards (with the Company Admin or appropriate Yard Admin roles).

Within each of these screens is a section labelled 'Account Preferences' which contains 2 drop down selections;

Selecting a 'Default Close Won Group' will automatically add a contact to the nominated group when you close a lead as 'Won'.

Selecting a 'Default Close Lost Group' will automatically add a contact to the nominated group when you close a lead as 'Lost'.

It is still possible to change the group when closing each individual lead - and you can also add additional groups as required.