fix: allow organizations to sign up with existing usernames (#25941)

* fix: allow organizations to sign up with existing usernames

- Update usernameCheck to check organization context when currentOrgDomain is provided
- Organizations can now use usernames that exist in other orgs or global namespace
- Only checks username availability within the specific organization
- Fixes issue where org signups were blocked by global username conflicts

Refs #25800

* Update username.ts

packages/lib/server/username.ts

@@ -1,5 +1,7 @@
 import type { NextResponse } from "next/server";
 
+import { ErrorCode } from "@calcom/lib/errorCodes";
+import { ErrorWithCode } from "@calcom/lib/errors";
 import slugify from "@calcom/lib/slugify";
 import prisma from "@calcom/prisma";
 import { RedirectType } from "@calcom/prisma/enums";
@@ -116,11 +118,36 @@ const usernameCheck = async (usernameRaw: string, currentOrgDomain?: string | nu
 
   const username = slugify(usernameRaw);
 
+  let organizationId: number | null = null;
+  if (currentOrgDomain) {
+    // Get organizationId from orgSlug
+    const organization = await prisma.team.findFirst({
+      where: {
+        isOrganization: true,
+        OR: [
+          { slug: currentOrgDomain },
+          { metadata: { path: ["requestedSlug"], equals: currentOrgDomain } },
+        ],
+      },
+      select: {
+        id: true,
+      },
+    });
+    if (!organization) {
+      throw new ErrorWithCode(
+        ErrorCode.NotFound,
+        `Organization with domain "${currentOrgDomain}" not found`,
+        { currentOrgDomain }
+      );
+    }
+    organizationId = organization.id;
+  }
+
   const user = await prisma.user.findFirst({
     where: {
       username,
-      // Simply remove it when we drop organizationId column
-      organizationId: null,
+      // Check in the specific organization context, or global namespace if no org
+      organizationId: organizationId ?? null,
     },
     select: {
       id: true,
@@ -140,12 +167,13 @@ const usernameCheck = async (usernameRaw: string, currentOrgDomain?: string | nu
     response.premium = true;
   }
 
-  // get list of similar usernames in the db
+  // get list of similar usernames in the db (scoped to organization if checking in org context)
   const users = await prisma.user.findMany({
     where: {
       username: {
         contains: username,
       },
+      ...(organizationId !== null ? { organizationId } : { organizationId: null }),
     },
     select: {
       username: true,