Gershom Bader wrote a book called "The Jewish Spiritual Heroes". It consists of multiple volumes and in volume 1 (The Creators of the Mishna), there is a chapter called "The Three Main Parties of the Talmudic Period; The Pharisees, The Sadducees, The Essenes". There it writes how a person became a member of the "Pharisee society". The society was not closed, and everyone could enter, provided they did what Bader writes below:
Any person could join if he promised, in the presence of three members, to observe the rules of the organization which concentrated on cleanliness and ablutions. It is interesting to note that in accepting a new member no distinction was made between a scholar and an uneducated person. Both were considered of equal worth and any person who was willing to assume the burdens of Pharisaism could join the society.
Furthermore, Bader writes:
As soon as a person became a member of the Pharisee society he undertook to serve G-d with his whole life. Such a person had to believe that G-d’s eye is all-seeing and that He observes all man’s acts in order to judge them for every injustice. Since man possesses a free will to do as he chooses he is held responsible for every act and he will have to account for his deeds before his Creator. The Pharisees also believed that God sometimes issues decrees governing nations or all of humanity and that such decrees may seem unreasonable to the victims. But such decrees are certainly just since G-d is just to the individual as well as to a group. If it appears that reward for good deeds or punishment for wrong doing is delayed, then one must remember that there is a reckoning after death, or later yet, when the dead will be resurrected, the deserving ones to eternal life and the evil ones to everlasting shame and suffering.
It is also worth noting what Bader writes on the meaning of the word Pharisee, which in Hebrew is translated as פרושים- "Perushim":
Since the Pharisees deeply understood the spirit of the Torah, they gave up the search for pleasure in life even as they avoided pride or any action which might seem immoral. They practiced brotherly love toward one another and each individual sought with all his might to avoid temptation.
"They gave up the search for pleasure" - an explanation of this can be found in the famous explanation on the Torah by Nachmanides, the Ramban.
------------ Side Note ----------------
The Ramban writes:
YE SHALL BE HOLY. “Abstain from the forbidden sexual relationships [mentioned in the preceding section] and from [other] sin, because wherever you find [in the Torah] a warning to guard against immorality, you find the mention of ‘holiness.’” This is Rashi’s language. But in the Torath Kohanim I have seen it mentioned without any qualification [i.e., without any particular reference to immorality, as Rashi expressed it], saying: “Be self-restraining.” Similarly, the Rabbis taught there: “And ye shall sanctify yourselves, and be ye holy, for I am Holy. Just as I am Holy, so be you holy. Just as I am Pure, so be you pure.”
The Ramban then connects this idea to the perushim:
And in my opinion, this abstinence does not refer only to restraint from acts of immorality, as the Rabbi [Rashi] wrote, but it is rather the self-control mentioned throughout the Talmud, which confers upon those who practice it the name of P’rushim (Pharisees), [literally: “those who are separated” from self-indulgence, as will be explained, or those who practice self-restraint].
The meaning thereof is as follows: The Torah has admonished us against immorality and forbidden foods, but permitted sexual intercourse between man and his wife, and the eating of [certain] meat and wine. If so, a man of desire could consider this to be a permission to be passionately addicted to sexual intercourse with his wife or many wives, and be among winebibbers, among gluttonous eaters of flesh,11 and speak freely all profanities, since this prohibition has not been [expressly] mentioned in the Torah, and thus he will become a sordid person within the permissible realm of the Torah!
See also this, where the Lubavitcher Rebbe writes:
והנה ידועה השאלה מהו תוכן הציווי קדושים תהיו והחידוש בו, כי הרי זהו ענין התורה בכללותה ומצותי' וענין כל אחד ואחד מציוויי התורה. והביאור בזה, אשר הציווי הוא בדבר כללי, שנהי' פרושים מן המותרות, אע"פ שלא הוזהרנו מהם בתורה ולא נאסרו עלינו, יזהיר שנהי' נקיים טהורים ופרושים מהם.
ולכאורה יאמר האדם ובפרט בעשותו חשבון צדק בנפשו ויעריך מצבו אליבי'
דנפשי' - דייך מה שאסרה תורה, והלואי שיקיים כל ציווי התורה המפורשים,
אלו שהן מדאורייתא, מדרבנן ועד לדקדוק קל של דברי סופרים וחומרות
האחרונים. ורק מי שכבר הגיע לדרגא זו - עליו להתעסק בעבודה הנעלית הזו -
פרושים מן המותרות.
[...] that we abstain from luxuries, even though we were not warned about them in the Torah and were not forbidden to us