diff --git a/lib/icinga/checkable.cpp b/lib/icinga/checkable.cpp
index 6bb251eef..eabce35e3 100644
--- a/lib/icinga/checkable.cpp
+++ b/lib/icinga/checkable.cpp
@@ -88,6 +88,18 @@ void Checkable::OnStateLoaded(void)
 	}
 }
 
+void Checkable::AddGroup(const String& name)
+{
+	boost::mutex::scoped_lock lock(m_CheckableMutex);
+
+	Array::Ptr groups = GetGroups();
+
+	if (!groups)
+		groups = make_shared<Array>();
+
+	groups->Add(name);
+}
+
 AcknowledgementType Checkable::GetAcknowledgement(void)
 {
 	ASSERT(OwnsLock());
diff --git a/lib/icinga/checkable.h b/lib/icinga/checkable.h
index 8d4f1c673..e068d2477 100644
--- a/lib/icinga/checkable.h
+++ b/lib/icinga/checkable.h
@@ -89,6 +89,8 @@ public:
 	std::set<Checkable::Ptr> GetParents(void) const;
 	std::set<Checkable::Ptr> GetChildren(void) const;
 
+	void AddGroup(const String& name);
+
 	//bool IsHostCheck(void) const;
 
 	bool IsReachable(DependencyType dt = DependencyState, shared_ptr<Dependency> *failedDependency = NULL, int rstack = 0) const;
@@ -270,6 +272,7 @@ protected:
 	virtual void OnStateLoaded(void);
 
 private:
+	mutable boost::mutex m_CheckableMutex;
 	bool m_CheckRunning;
 	long m_SchedulingOffset;
 
diff --git a/lib/icinga/checkable.ti b/lib/icinga/checkable.ti
index dd2fade39..380ff9ff2 100644
--- a/lib/icinga/checkable.ti
+++ b/lib/icinga/checkable.ti
@@ -1,5 +1,6 @@
 #include "icinga/icingaapplication.h"
 #include "base/dynamicobject.h"
+#include "base/array.h"
 
 namespace icinga
 {
@@ -20,7 +21,9 @@ enum AcknowledgementType
 
 abstract class Checkable : DynamicObject
 {
-	[config] Array::Ptr groups;
+	[config] Array::Ptr groups {
+		default {{{ return make_shared<Array>(); }}}
+	};
 	[config, protected] String check_command (CheckCommandRaw);
 	[config] int max_check_attempts (MaxCheckAttemptsRaw) {
 		default {{{ return 3; }}}
diff --git a/lib/icinga/host.ti b/lib/icinga/host.ti
index a45493258..1a8c3ed83 100644
--- a/lib/icinga/host.ti
+++ b/lib/icinga/host.ti
@@ -14,7 +14,6 @@ class Host : Checkable
 				return m_DisplayName;
 		}}}
 	};
-	[config] Array::Ptr groups;
 
 	[config] String address;
 	[config] String address6;
diff --git a/lib/icinga/user.cpp b/lib/icinga/user.cpp
index 6834574bf..ca29c2935 100644
--- a/lib/icinga/user.cpp
+++ b/lib/icinga/user.cpp
@@ -69,6 +69,18 @@ void User::Stop(void)
 	}
 }
 
+void User::AddGroup(const String& name)
+{
+	boost::mutex::scoped_lock lock(m_UserMutex);
+
+	Array::Ptr groups = GetGroups();
+
+	if (!groups)
+		groups = make_shared<Array>();
+
+	groups->Add(name);
+}
+
 TimePeriod::Ptr User::GetPeriod(void) const
 {
 	return TimePeriod::GetByName(GetPeriodRaw());
diff --git a/lib/icinga/user.h b/lib/icinga/user.h
index d5b441ca6..780cd979f 100644
--- a/lib/icinga/user.h
+++ b/lib/icinga/user.h
@@ -40,6 +40,8 @@ public:
 	DECLARE_PTR_TYPEDEFS(User);
 	DECLARE_TYPENAME(User);
 
+	void AddGroup(const String& name);
+
 	/* Notifications */
 	TimePeriod::Ptr GetPeriod(void) const;
 
@@ -52,6 +54,8 @@ protected:
 	virtual void Stop(void);
 
 	virtual void OnConfigLoaded(void);
+private:
+	mutable boost::mutex m_UserMutex;
 };
 
 }
diff --git a/lib/icinga/user.ti b/lib/icinga/user.ti
index 2c25a7631..052c16151 100644
--- a/lib/icinga/user.ti
+++ b/lib/icinga/user.ti
@@ -1,4 +1,5 @@
 #include "base/dynamicobject.h"
+#include "base/array.h"
 
 namespace icinga
 {
@@ -13,7 +14,9 @@ class User : DynamicObject
 				return m_DisplayName;
 		}}}
 	};
-	[config] Array::Ptr groups;
+	[config] Array::Ptr groups {
+		default {{{ return make_shared<Array>(); }}}
+	};
 	[config] String period (PeriodRaw);
 	[config] Array::Ptr types;
 	int type_filter_real (TypeFilter);