Csdrhrt

How can I unmask the e-mail addresses in a Bcc field when I am just a recipient?

Need very simple, step-by-step instructions for someone who doesn't code. I have received a group e-mail and would really like to see the others who got it.

You can't. You simply won't have any information about the Bcc header when you receive the mail, so you there's nothing to "unmask".

The way Bcc is designed is specified in RFC 2822, under section 3.6.3. To quote the specification:

The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains
addresses of recipients of the message whose addresses are not to be
revealed to other recipients of the message. There are three ways in
which the "Bcc:" field is used. In the first case, when a message
containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is
removed even though all of the recipients (including those specified
in the "Bcc:" field) are sent a copy of the message. In the second
case, recipients specified in the "To:" and "Cc:" lines each are sent
a copy of the message with the "Bcc:" line removed as above, but the
recipients on the "Bcc:" line get a separate copy of the message
containing a "Bcc:" line. (When there are multiple recipient
addresses in the "Bcc:" field, some implementations actually send a
separate copy of the message to each recipient with a "Bcc:"
containing only the address of that particular recipient.) Finally,
since a "Bcc:" field may contain no addresses, a "Bcc:" field can be
sent without any addresses indicating to the recipients that blind
copies were sent to someone. Which method to use with "Bcc:" fields
is implementation dependent, but refer to the "Security
Considerations" section of this document for a discussion of each.

When a message is a reply to another message, the mailboxes of the
authors of the original message (the mailboxes in the "From:" field)
or mailboxes specified in the "Reply-To:" field (if it exists) MAY
appear in the "To:" field of the reply since these would normally be
the primary recipients of the reply. If a reply is sent to a message
that has destination fields, it is often desirable to send a copy of
the reply to all of the recipients of the message, in addition to the
author. When such a reply is formed, addresses in the "To:" and "Cc:"
fields of the original message MAY appear in the "Cc:" field of the
reply, since these are normally secondary recipients of the reply. If
a "Bcc:" field is present in the original message, addresses in that
field MAY appear in the "Bcc:" field of the reply, but SHOULD NOT
appear in the "To:" or "Cc:" fields.

Note: Some mail applications have automatic reply commands that
include the destination addresses of the original message in the
destination addresses of the reply. How those reply commands behave
is implementation dependent and is beyond the scope of this document.
In particular, whether or not to include the original destination
addresses when the original message had a "Reply-To:" field is not
addressed here.

In practice the case where To and Cc recipients receive no Bcc line, but each Bcc'ed address receives a Bcc line containing only their email address, is most common. This provides no indication of a Bcc to the To and Cc recipients, and indicates to the Bcc'ed recipients that they were sent the email via the use of Bcc without revealing other Bcc recipients.

Typically not possible if you don't have control over the sender SMTP server since this field is not transmitted to the recipient SMTP server.

When sending a mail, the sender SMTP server checks the BCC field and creates a copy for each recipient listed, removing the list of other recipients.
That is the whole point of BCC functionality.

Request For Comments (RFC) standard (published by The Internet Engineering Task Force (IETF)) specifies that recipients of an email, sent to recipients specified in "BCC" header may receive the email but not be aware of any other recipients mentioned in the header. Specifically, "addresses are not to be revealed to other recipients of the message".

It's a request to SMTP servers to reflect current practice (protocol) for the Internet community by The Internet Society.

Those found to be not compliant may be segregated and if found to be rogue, will be banned/blacklisted, and even prosecuted when found to conduct activities in contravention of laws in the jurisdiction.

So if you're a recipient of an email from a compliant (mail) server, you won't receive other recipients emails mentioned in the "BCC" field, unless you're in control of the sending (SMTP) server, the incoming (POP,IMAP, etc) server, and all the relay servers that routed the IP packets.