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1. Be a member of a guild with at least 2 users, including yourself.
2. Confirm that the other user is not your friend.
3. Confirm that the option that allows you to add people in a group DMs, who are not your friends, is enabled in the other user's Settings.

("Privacy -> Friends & direct messages -> Communication -> Group chats -> Anyone can add you to group chats")

4. Open Direct Messages modal window within your profile.
5. Observe that only friends are displayed, and users belonging to the same guild cannot be added, they are not displayed in the search list, and cannot be searched, despite the enabled setting.

It should be possible to add any non-blocked user to a group DM, not only mutual friends.

The user picker modal only shows friends. Non-friends (including people from the same guilds or users with whom I already have a DM history) cannot be searched for or selected.