Abstain from all or some kinds of food or drink, especially as a religious observance. Oxford Dictionary
Fasting is a willing abstinence or reduction from some or all food, drink, or both, for a period of time. An absolute fast (dry fasting) is normally defined as abstinence from all food and liquid for a defined period, usually a period of 24 hours, or several days. Water fasting allows drinking water but nothing else. Other fasts may be partially restrictive, limiting only particular foods or substances. A fast may also be intermittent in nature. Fasting practices may preclude intercourse and other activities as well as food. Wikipedia, the free encyclopedia
There are many preachers today who wrongly teach fasting from the Old Testament. Fasting is a personnel decision and must be kept between the person and God.
Although not commanded of us by God, if done correctly and sincerely, I can’t see why God would not honor the humility of the fast.
Group fasts would be the same as the Pharisees hypocritical pretenses. Fasting is an individual decision and is between the person and God. What good would it do to announce your fast to the church? A Fast is about humility and a closeness to God. An announcement would be a statement of pride and could give the wrong message to both the church and to God.