diff --git a/lib/private/Share20/Manager.php b/lib/private/Share20/Manager.php
index 13e2b55fe2ba4..d9fee81871804 100644
--- a/lib/private/Share20/Manager.php
+++ b/lib/private/Share20/Manager.php
@@ -157,14 +157,15 @@ protected function verifyPassword(?string $password): void {
 	 * @suppress PhanUndeclaredClassMethod
 	 */
 	protected function generalChecks(IShare $share): void {
+		$shareWith = $share->getSharedWith();
 		if ($share->getShareType() === IShare::TYPE_USER) {
 			// We expect a valid user as sharedWith for user shares
-			if (!$this->userManager->userExists($share->getSharedWith())) {
+			if ($shareWith === null || !$this->userManager->userExists($shareWith)) {
 				throw new \InvalidArgumentException($this->l->t('Share recipient is not a valid user'));
 			}
 		} elseif ($share->getShareType() === IShare::TYPE_GROUP) {
 			// We expect a valid group as sharedWith for group shares
-			if (!$this->groupManager->groupExists($share->getSharedWith())) {
+			if ($shareWith === null || !$this->groupManager->groupExists($shareWith)) {
 				throw new \InvalidArgumentException($this->l->t('Share recipient is not a valid group'));
 			}
 		} elseif ($share->getShareType() === IShare::TYPE_LINK) {
diff --git a/lib/private/User/Manager.php b/lib/private/User/Manager.php
index 81fe55db58e1d..c72d5f2e91442 100644
--- a/lib/private/User/Manager.php
+++ b/lib/private/User/Manager.php
@@ -118,25 +118,29 @@ public function clearBackends(): void {
 	}
 
 	/**
-	 * get a user by user id
-	 *
-	 * @param string $uid
-	 * @return User|null Either the user or null if the specified user does not exist
+	 * {@inheritDoc}
+	 * @param list<string> $excludeBackends A list of IUserBackend::getBackendName() that need to be excluded from the search.
 	 */
 	#[\Override]
-	public function get($uid) {
+	public function get($uid, array $excludeBackends = []): ?\OCP\IUser {
 		if (is_null($uid) || $uid === '' || $uid === false) {
 			return null;
 		}
-		if (isset($this->cachedUsers[$uid])) { //check the cache first to prevent having to loop over the backends
-			return $this->cachedUsers[$uid];
-		}
 
 		if (strlen($uid) > IUser::MAX_USERID_LENGTH) {
 			return null;
 		}
 
+		// check the cache first to prevent having to loop over the backends
+		if ($excludeBackends === [] && isset($this->cachedUsers[$uid])) {
+			return $this->cachedUsers[$uid];
+		}
+
 		$cachedBackend = $this->cache->get(sha1($uid));
+		if (in_array($cachedBackend, $excludeBackends)) {
+			$cachedBackend = null;
+		}
+
 		if ($cachedBackend !== null && isset($this->backends[$cachedBackend])) {
 			// Cache has the info of the user backend already, so ask that one directly
 			$backend = $this->backends[$cachedBackend];
@@ -151,6 +155,10 @@ public function get($uid) {
 				continue;
 			}
 
+			if (in_array($i, $excludeBackends)) {
+				continue;
+			}
+
 			if ($backend->userExists($uid)) {
 				// Hash $uid to ensure that only valid characters are used for the cache key
 				$this->cache->set(sha1($uid), $i, 300);
@@ -193,19 +201,13 @@ public function getUserObject($uid, $backend, $cacheUser = true) {
 		return $user;
 	}
 
-	/**
-	 * check if a user exists
-	 *
-	 * @param string $uid
-	 * @return bool
-	 */
 	#[\Override]
-	public function userExists($uid) {
+	public function userExists(string $uid, array $excludeBackends = []): bool {
 		if (strlen($uid) > IUser::MAX_USERID_LENGTH) {
 			return false;
 		}
 
-		$user = $this->get($uid);
+		$user = $this->get($uid, $excludeBackends);
 		return ($user !== null);
 	}
 
diff --git a/lib/public/IUserManager.php b/lib/public/IUserManager.php
index f19685903baaf..0cd6daa399e49 100644
--- a/lib/public/IUserManager.php
+++ b/lib/public/IUserManager.php
@@ -69,7 +69,7 @@ public function clearBackends();
 	 * @return \OCP\IUser|null Either the user or null if the specified user does not exist
 	 * @since 8.0.0
 	 */
-	public function get($uid);
+	public function get($uid): ?\OCP\IUser;
 
 	/**
 	 * Get the display name of a user
@@ -81,13 +81,14 @@ public function get($uid);
 	public function getDisplayName(string $uid): ?string;
 
 	/**
-	 * check if a user exists
+	 * Check if a user exists.
 	 *
 	 * @param string $uid
+	 * @param list<string> $excludeBackends A list of IUserBackend::getBackendName() that need to be excluded from the search.
 	 * @return bool
 	 * @since 8.0.0
 	 */
-	public function userExists($uid);
+	public function userExists(string $uid, array $excludeBackends = []): bool;
 
 	/**
 	 * Check if the password is valid for the user
diff --git a/tests/Core/Command/User/AddTest.php b/tests/Core/Command/User/AddTest.php
index 59ce5ca0f908c..43b7aaea55601 100644
--- a/tests/Core/Command/User/AddTest.php
+++ b/tests/Core/Command/User/AddTest.php
@@ -109,6 +109,11 @@ public function testAddEmail(
 		$this->mailHelper->expects($isEmailValid && $shouldSendEmail ? static::once() : static::never())
 			->method('sendMail');
 
+		$this->consoleInput->method('getArgument')
+			->willReturnMap([
+				['uid', 'JohnDoe'],
+			]);
+
 		$this->consoleInput->method('getOption')
 			->willReturnMap([
 				['generate-password', 'true'],
diff --git a/tests/lib/Share20/ManagerTest.php b/tests/lib/Share20/ManagerTest.php
index 52d98569e2ff5..4c30716bf5a8c 100644
--- a/tests/lib/Share20/ManagerTest.php
+++ b/tests/lib/Share20/ManagerTest.php
@@ -1195,10 +1195,7 @@ public function testGeneralChecks(array $shareParams, ?string $exceptionMessage,
 
 		$thrown = null;
 
-		$this->userManager->method('userExists')->willReturnMap([
-			['user0', true],
-			['user1', true],
-		]);
+		$this->userManager->method('userExists')->willReturnCallBack(fn (string $userId) => $userId === 'user0' || $userId === 'user1');
 
 		$user0 = $this->createMock(IUser::class);
 		$user0