Diese Leute sind meine nette Freunde
Or
Diese Leute sind meine netten Freunde
And why?
Diese Leute sind meine nette Freunde
Or
Diese Leute sind meine netten Freunde
And why?
It should be: Diese Leute sind meine netten Freunde.
Attributive adjectives (''nett'') change their ending together with the noun they refer to (''Freunde''). The way they change also depends on whether they are used together with definite articles, with indefinite articles/possessive determiners or without any articles. You can find the respective endings in this table.
Here we have nominative, plural (''die Freunde'') and a possessive determiner/Possessivartikel (''mein''). The ending is therefore -en.
Without the possessive determiner it would be:
Diese Leute sind nette Freunde.
Diese Leute sind meine netten Freunde.
This sentence follows the A is B pattern, it's a so called Gleichsetzungsnominativ. Both diese Leute and meine netten Freunde is in the nominative case.
German adjective declension differs from noun declension. It has less forms but depends on whether the definite article/demonstrative pronoun, the indefinite article/possessive pronoun, or no article/pronoun at all prefixes the adjective row.