Get Group Details from Person or Group field using REST API?

Try using this:

_api/web/lists/getbytitle('listname')/items(100)?$select=Initiator/EMail,RAReviewers/EMail&$expand=Initiator,RAReviewers

Above is working fine for me in case of person or group.

Example:

/_api/web/lists/getbytitle('TestList')/items(1)?$select=TestPersonField/Title,TestPersonField/EMail&$expand=TestPersonField

Output: Below is part of response returned from above API call

<entry>
    <content type="application/xml">
        <m:properties>
            <d:Title>Ganesh Sanap</d:Title>
            <d:EMail>gsanap@tenant.onmicrosoft.com</d:EMail>
        </m:properties>
    </content>
</entry>
<entry>
    <content type="application/xml">
        <m:properties>
            <d:Title>SPConnect Members</d:Title>
            <d:EMail>SPConnect@tenant.onmicrosoft.com</d:EMail>
        </m:properties>
    </content>
</entry>

Where SPConnect Members is a Microsoft 365 group name.

Note: This will not work for SharePoint groups as they don’t have Email property.