1. How many times a day can one say a beracha on a rainbow?
2. Is one able to inform someone else that there is a rainbow so that they should be able to say a beracha?
1. The blessing can be made more than once a day, provided that one sees a different rainbow. On the same rainbow, one makes a blessing only once (based on Magen Avraham 218:5; Machazik Berachah 229; Shaarei Teshuvah and Mishnah Berurah).
2. One should not inform another person. Most write that it is “improper” to do so, whereas Kaf Hachaim writes that it is actually prohibited(!). See Chayei Adam 63:4; Salmas Chaim 142; Kaf Hachaim and Mishnah Berurah 229:1.