(agreeing): While used somewhat like an adjective in that it can modify a noun, de acuerdo is in fact a prepositional phrase, and as may be seen in the above example sentences, it does not inflect for gender or number.
(agreeing): As may be seen in the above example sentences, de acuerdo frequently appears with the verb estar(“to be”): “estar de acuerdo” means “to be in agreement, to agree” (but is always stative).
(agreeing): The nominal modified by de acuerdo may refer either to both agreeing parties (as in the first example above), or only to one (as in the second and third). In the latter case, the other agreeing party may be left implicit, or may be expressed using the preposition con(“with”) (as in the second example above); or, con may introduce a claim with which the nominal agrees (as in the third).
(agreeing): It is also used as an interjection (as in the fourth example above) to indicate acceptance or accord, very much like English "all right" or "OK".