diff --git a/monggregate/stages/group.py b/monggregate/stages/group.py
index 53ad66b..05ea3c9 100644
--- a/monggregate/stages/group.py
+++ b/monggregate/stages/group.py
@@ -66,7 +66,7 @@
 from typing import Any
 from monggregate.base import pyd
 from monggregate.stages.stage import Stage
-from monggregate.utils import validate_field_path
+from monggregate.utils import validate_field_path, validate_field_paths
 
 class Group(Stage):
     """
@@ -91,6 +91,7 @@ class Group(Stage):
     # Validators
     # ------------------------------------------
     _validate_by = pyd.validator("by", pre=True, always=True, allow_reuse=True)(validate_field_path) # re-used pyd.validator
+    _validate_iterable_by = pyd.validator("by", pre=True, always=True, allow_reuse=True)(validate_field_paths) # re-used pyd.validator
 
     @pyd.validator("query", always=True)
     @classmethod
diff --git a/monggregate/utils.py b/monggregate/utils.py
index 4ba419d..e38c882 100644
--- a/monggregate/utils.py
+++ b/monggregate/utils.py
@@ -66,7 +66,18 @@ def to_unique_list(keys:T)->list[str]|T:
 def validate_field_path(path:str|None)->str|None:
     """Validates field path"""
 
-    if path and not path.startswith("$"):
+    if isinstance(path, str) and not path.startswith("$"):
         path =  "$" + path
 
     return path
+
+
+def validate_field_paths(paths:list[str]|set[str])->list[str]:
+    """Validates field paths"""
+
+    if isinstance(paths, list):
+        paths = [validate_field_path(path) for path in paths]
+    elif isinstance(paths, set):
+        paths = {validate_field_path(path) for path in paths}
+
+    return paths
diff --git a/test/test_stages.py b/test/test_stages.py
index 6b52310..d1d371f 100644
--- a/test/test_stages.py
+++ b/test/test_stages.py
@@ -115,6 +115,7 @@ def test_count(self, state:State)->None:
         assert count
         state["count"] = count
 
+
     def test_group(self, state:State)->None:
         """Tests the group stage"""
 
@@ -141,6 +142,52 @@ def test_group(self, state:State)->None:
             }
         )
 
+        # Test by as list
+        # ------------------------
+        assert Group(
+            by=["name", "age"],
+            query = {
+                "output":{"$sum":"income"}
+            }
+        )
+
+        # Test by as set
+        # ------------------------
+        assert Group(
+            by=set(["name", "age"]),
+            query = {
+                "output":{"$sum":"income"}
+            }
+        )
+
+        # Test by as constant
+        # ------------------------
+        assert Group(
+            by=1,
+            query = {
+                "output":{"$sum":"income"}
+            }
+        )
+
+        # Test by as dict
+        # ------------------------
+        assert Group(
+            by={"name":"$name"},
+            query = {
+                "output":{"$sum":"income"}
+            }
+        )
+
+        # Test by as None
+        # ------------------------
+        assert Group(
+            by=None,
+            query = {
+                "output":{"$sum":"income"}
+            }
+        )
+        
+
 
     def test_limit(self, state:State)->None:
         """Tests the limit stage"""