diff -u b/src/GroupMembership.php b/src/GroupMembership.php
--- b/src/GroupMembership.php
+++ b/src/GroupMembership.php
@@ -86,7 +86,7 @@
    */
   protected function getGroupTypeRoles() {
     $properties = [
-      'group_type' => $this->groupContent->getGroup()->getGroupType()->id(),
+      'group_type' => $this->groupContent->getGroup()->bundle(),
       'internal' => FALSE,
     ];
 
@@ -139,26 +139,27 @@
   }
 
   /**
-   * Add a role.
+   * Add a role to a member.
    *
    * @param string $role
-   *    Role id.
+   *   The role id for the role to add.
    */
   public function addRole($role) {
-    // Check that role exists for a given group type.
+    // Check that the role exists for a given group type.
     $group_type_roles = $this->getGroupTypeRoles();
     $role_exists = FALSE;
 
     foreach (array_keys($group_type_roles) as $group_type_role) {
-      if ($group_type_role == $role) {
+      if ($group_type_role === $role) {
         $role_exists = TRUE;
+        break;
       }
     }
 
     if ($role_exists) {
-      // Check that a user has already the role.
+      // Check if a member already has the role.
       foreach ($this->groupContent->group_roles as $key => $role_item) {
-        if ($role_item->target_id == $role) {
+        if ($role_item->target_id === $role) {
           return;
         }
       }
@@ -169,10 +170,10 @@
   }
 
   /**
-   * Remove a role.
+   * Remove a role from a member.
    *
    * @param string $role
-   *    Role id.
+   *   The role id for the role to remove.
    */
   public function removeRole($role) {
     $roles = $this->groupContent->group_roles;
only in patch2:
unchanged:
--- /dev/null
+++ b/tests/src/Kernel/GroupMembershipTest.php
@@ -0,0 +1,82 @@
+<?php
+
+namespace Drupal\Tests\group\Kernel;
+
+/**
+ * Tests the general behavior of group membership.
+ *
+ * @coversDefaultClass \Drupal\group\GroupMembership
+ * @group group
+ */
+class GroupMembershipTest extends GroupKernelTestBase {
+
+  /**
+   * The group we will use to test methods on.
+   *
+   * @var \Drupal\group\Entity\Group
+   */
+  protected $group;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->group = $this->createGroup();
+  }
+
+  /**
+   * Tests the addition of a role to a member of a group.
+   *
+   * @covers ::addRole
+   */
+  public function testAddRole() {
+    $account = $this->createUser();
+    $this->group->addMember($account);
+    $member = $this->group->getMember($account);
+    $memberRoles = $member->getRoles();
+    $this->assertArrayNotHasKey('default-custom', $memberRoles);
+
+    // Verify adding a non-existing role on a member that didn't had that role
+    // previously, doesn't add the role.
+    $member->addRole('default-non-existing-role');
+    $memberRoles = $member->getRoles();
+    $this->assertArrayNotHasKey('default-non-existing-role', $memberRoles);
+
+    // Verify adding an existing role on a member that has that role already,
+    // doesn't set the role twice.
+    $member->addRole('default-member');
+    $memberRoles = $member->getRoles();
+    $this->assertArrayHasKey('default-member', $memberRoles);
+
+    // Verify adding an existing role on a member that didn't had that role
+    // previously, adds the role.
+    $member->addRole('default-custom');
+    $memberRoles = $member->getRoles();
+    $this->assertArrayHasKey('default-custom', $memberRoles);
+  }
+
+  /**
+   * Tests the removing of a role on a member of a group.
+   *
+   * @covers ::removeRole
+   */
+  public function tesRemoveRole() {
+    $account = $this->createUser();
+    $this->group->addMember($account);
+    $member = $this->group->getMember($account);
+    $memberRoles = $member->getRoles();
+    $this->assertArrayNotHasKey('default-custom', $memberRoles);
+
+    // First add a role we can remove afterwards.
+    $member->addRole('default-custom');
+    $memberRoles = $member->getRoles();
+    $this->assertArrayHasKey('default-custom', $memberRoles);
+
+    // Now remove that role again.
+    $member->removeRole('default-custom');
+    $memberRoles = $member->getRoles();
+    $this->assertArrayNotHasKey('default-custom', $memberRoles);
+  }
+
+}