Who are "Shomerei ha'Bris"?
Radak: It refers to the children of Yere'av (verse 17). When they are good, they receive their reward and the reward of their parents - "v'Oseh Chesed la'Alafim l'Ohavai ul'Shomerei Mitzvosai" (Shemos 20:6). And even when the children are not good, Hashem benefits them due to their fathers' merits - "v'Zacharti Lahem Bris Rishonim" (Vayikra 26:45).
Radak citing Ibn Ezra: They are people who fear Hashem.
Radak citing Shocher Tov: This refers to Yisrael in the Midbar. Hashem supplied for them manna and the well, and they could guard the Torah and exert in it. However, now that man needs to work for his income, they remember His Mitzvos to do them (but they cannot exert in them all the time). A parable for this is a widow who married off her son. She told him, I know that now you cannot attend to me like you used to. However, remember me!
Who are "Zocherei Fikudav La'asosam"?
Radak: They mention [His Mitzvos] verbally and learn them in order to do them. Learning without fulfilling is not proper.
Radak citing Ibn Ezra: They are Ohavav. Hashem's commands are always in front of them and in their hearts; they do not forget for a moment.
Radak citing Shocher Tov: Refer to 103:18:1:3.
