Greeting a person before Davening:[1]

• Going to another’s house for the purpose of greeting him: From when the time of Alos arrives until one Davens Shemoneh Esrei, it is forbidden for one to go to another person’s house specifically for the purpose of greeting him, such as to tell him “Shalom Aleiychem” or “good morning.” This applies even if one has already said the morning blessings, Pesukei Dezimra, and Kerias Shema.
• If one is going to the person’s house for other purposes: If one is in any event passing by another person’s home prior to Davening, such as for a business purpose, then it is permitted to stop by his home and wish him good morning. One may not however greet him with the words “Shalom Aleiychem”, as Shalom is one of the names of G-d.
• Exchanging greetings with a person one meets: If one was on his way to Shul or the marketplace and the like, and he happened to meet up with an acquaintance, whether on the way or at the destination, then he may greet him with “good morning,” and from the letter of the law may tell him “Shalom Aleiychem.” Nevertheless, the custom is even in such a case to avoid saying the word “Shalom,” and rather to say good morning or a statement of the like.
• May one call someone prior to Davening? If one is calling him for the purpose of greeting him, then it is forbidden to do so prior to prayer. If, however, one needs to call him for another purpose, then one may do so, and is allowed to begin the conversation with “good morning” greetings, although not with saying the words “Shalom Aleiychem.”
• Wishing Shabbat Shalom on Shabbos before Shacharis: Being that we do not say the word shalom prior to Davening Shacharis as explained above, therefore instead of saying “Shabbat Shalom” in the morning prior to prayers, one should say “good Shabbos” and the like. Unfortunately, many people are unaware of this.

[1] See Admur 89:3; Michaber 89:2; Rebbe Aba Brachos 14a; Rashba Brachos 14a; M”B 89:16; Kaf Hachaim 89:18 and 21; Ketzos Hashulchan 11:1; Piskeiy Teshuvos 89:11; Elya Raba 307:3 in name of Shelah p. 135; Beir Heiytiv 89:3