Many misconceptions here. Plebs =/= Plebeians (in the strict sense for there was of course overlap) and both Augustus (as Octavius, before his adoption by Caesar) and Agrippa were of Plebeian families. Both also belonged to the Roman nobilty, later called Nobiles, which had (starting in the 4th century BC) been opened up for Plebeian families and very soon became flooded with them as the wealth required to move up in society more and more became accessible as the Roman empire^1 expanded. This influx was the more impactful as the number of Patrician families would steadily decline over time. The dichotomy of the early Roman republic of Patricians vs Plebeians was replaced by the dichotomy of Nobiles + Equites vs Plebs; the new order had been firmly solidified by the last quarter of the 4th century BC if not earlier. The Equites and a large part of the Nobiles (within a century the majority) were comprised of wealthy Plebeians. The laws enacted during the course of the 4th century allowed for Plebeians to be eligible for every political office (thus giving them seats in the senate as well, as it was made up of ex-magistrates) and one law even specified that each year at least 1 of the 2 consuls had to be a Plebeian (by the late republic there were streaks of several consecutive years with only Plebeian consuls). To belong to the Nobiles you had to have the wealth required for the top orders of society (as decided by the recurring censuses) and you would have to have distinguished ancestors that had served as magistrates and senators. The only significant distinctions remaining between Patricians and Plebeians were that the former were still ineligible for the office of tribune of the plebs and certain priestly offices were barred for Plebeians.

Note 1: following Finlay I use 'Roman empire' to denote the large territorial extent of the state of Republic Rome and 'Roman Empire' to denote the government which replaced the republic as well as the territorial extent of that government.