Dear Andrea,

You have excellent English and it took me a little time to work you were German because Koch is a common enough surname in North Amreica. "Select" is an adjective used descriptively with the plural noun "partners". Selected is the past participle of the verb "to select" and is operating here as an adjective too. There is one particular nuance in meaning, selected implies something already done; select has an implication of something special about the partners. Perhaps select partners is marginally more flattering.

Apologies for the English lesson.

David