Recipients....
You can usually tell if a message is a group message by looking for multiple recipients listed in the "To" field or by seeing replies from others in the conversation. In some messaging apps, group messages may be indicated by a specific icon or label, such as "Group Chat." Additionally, if you receive responses from multiple people rather than just one, it's likely a group message.
The group which does not receive experimental treatment is the control group, the group which does receive the treatment is the experimental group.
When a person speaks to an audience, it is called public speaking. It involves delivering a message or speech to a group of people in a public setting.
article
A message told out loud to a group of people
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A group with nine people is called a nonet.
A group of people is called a crowd
a group or team of intelligent people are called wath
It's called a group.
An investigation in which a group that receives some experimental treatment is compared to a group that does not receive the experimental treatment can be called a placebo-controlled study or a comparative experiment, both of which are types of clinical studies. The group receiving the experimental treatment is called the treatment group, and the group that is not receiving the experimental treatment is called the control group.