How do you spread joy up to the maximum & bring gloom down to the minimum?

This is easy. You become joyful and do not worry about the maximum or minimum. A joyful person will meet your joy with joy of his own. A miserable person will remain miserable in the face of joy. There is no need to try and convince them to see life the way you see it. Leave the door open, invite them along, but do nothing more. Your attempt to "give them joy" will result in them hearing the message, "He wants me to change." The message you give them is that they are OK to stay where they are or OK to come along and it is totally up to them. You spread joy by example and not by trying to convince, urge, prompt, or cajole others into being joyful.