Why would there be ethical issues of risk associated with a researcher becoming part of a group without telling group members that they were being studied?

What will be an ideal response?

If a researcher joined a group without telling the members that they were being studied, those members might form friendships with the researcher and feel violated when they discovered the truth. In addition, the people in the group might engage in behaviors that they didn't feel comfortable doing in the presence of an outsider, so their privacy may be invaded.