WhatsApp group privacy comes down to one fact: every member of the group can see your phone number, and no setting hides it from them. Telegram lets a username stand in for your number. Discord shows other members a username, not a phone number.
Short answerWhatsApp: your number is visible to everyone in the group, admins and strangers alike.Telegram: your number can stay hidden if you set the privacy option and use a username.Discord: others see your username, avatar and profile, not your phone number.
WhatsApp group privacy phone number: what every member sees
The moment you join a WhatsApp group, by link or by being added, your number appears in the member list. Everyone in the group sees it, whether or not they have you in their contacts, and so do the admins. There is no toggle in Settings that changes this for groups. It is how the app identifies people, since an account is a phone number.
That is worth thinking about before you tap Join on a large public group. Joining through an invite link feels casual, but you are handing your number to every current member and every future one. The practical steps for joining are in how to join a WhatsApp group with a link, and the number question belongs in your decision before step one.
Some things you can limit. In Settings, Privacy, you choose who sees your profile photo, your About line and your last seen. Setting them to My contacts means strangers in a group see the name you chose and not much else. You can also control who is allowed to add you to groups, which stops random adds, though not the exposure that comes from joining a group yourself.
Messages in WhatsApp groups are end-to-end encrypted. That protects them from people outside the chat. It does nothing about the members inside, who can read, screenshot and forward everything you write. The WhatsApp Help Centre has the current privacy options if you want to see them for yourself, and the app is the better source if WhatsApp changes how this works.
Telegram: a username can stand in for your number
Telegram works differently, because an account can have a username. In a group, other members see your display name, your photo and your username if you set one. Your phone number is controlled by a setting in Privacy and Security, where you choose who can see it. Check that setting rather than trusting the default. Setting it to nobody hides the number from people who do not already have it saved.
There is a catch. A second setting decides who can find you by your number, and if someone already has your number and you allow it, they can still look you up. Also, a username makes you reachable by anyone who spots it in a public group. That trades one exposure for another, and the trade is usually worth it, but it is a trade.
Telegram groups and channels are not end-to-end encrypted in the way WhatsApp groups are, as the Telegram FAQ explains. A bot added to a group can also read messages depending on how it is set up. Joining by invite link is covered in how to join a Telegram group with an invite link.
Discord shows a username, not a number
Discord is built around usernames, so a server shows your username, display name, avatar and whatever profile details you chose to add. Your phone number, if you gave one for verification, is not displayed to other members. A server can also see roughly when you joined and which roles you hold.
Discord is not private in other ways. Your profile can show accounts you have connected, and members can sometimes see which servers you have in common. Messages in a server are visible to everyone who can read that channel. Discord's own page on joining a server explains how invites work, and the privacy settings in your account let you decide who may message you directly after you share a server.
Check what a stranger sees before you join
Do this once, on each app you use. In WhatsApp, open Settings, then Privacy, and look at who can see your photo, About and last seen. Ask a friend who does not have your number saved to search for you and tell you what shows. In Telegram, open Settings, then Privacy and Security, and read the phone number options line by line.
Your display name is visible everywhere, so pick one you would be fine with a stranger reading. Many people use a full name on WhatsApp out of habit, and in a public group that is more than a stranger needs.
Side by side
Where your details travel after the group
A number in a big public group is easy for anyone to collect. Spam calls, unwanted adds and messages from strangers are the common result, and scam accounts often start with a polite hello that references the group you both belong to. How those approaches usually go is in the signs a group link is not what it claims.
Leaving a group does not delete what you posted, and it does not take back the number members already saw. Screenshots outlive everything.
If you only do one thing
Before joining any large public WhatsApp group, decide if you are happy for every member to hold your number. If you are not, use a separate number for public groups, or take the same topic to a Telegram group where a username does the job. The wider picture of how these invites circulate is in how group links work.