diff --git a/src/fiber_package/groups/communication.py b/src/fiber_package/groups/communication.py
new file mode 100644
index 0000000..13c3d24
--- /dev/null
+++ b/src/fiber_package/groups/communication.py
@@ -0,0 +1,74 @@
+from pydantic import BaseModel, model_validator, Field, HttpUrl
+from enum import StrEnum
+from typing import Annotated, Self
+from fiber_package.util import GroupRef
+from datetime import datetime
+
+
+# may be moved to another file if used elsewhere
+'''
+The category is only used to place the communication correctly in the weekly
+mailing and the board-website, it is not visible (or relevant) to readers
+'''
+class CommunicationCategory(StrEnum):
+    announcement = "announcement"
+    news = "news"
+    activity = "activity"
+
+
+'''
+A "communication" is a piece of communication/promotion to be shared with
+members, which may appear on the website and in the weekly mailing.
+'''
+class CommunicationModel(BaseModel):
+    category: CommunicationCategory
+
+    name_en: str
+    name_nl: str
+
+    subtitle_en: str
+    subtitle_nl: str
+    # subtitles currently (2026) appear only in the table of contents in the
+    # weekly mailing; they are meant to be a short catchy flavour text
+
+    description_en: str
+    description_nl: str
+
+    image: HttpUrl | None
+    # often, a poster comes associated with a communication (in particular if
+    # it's an activity). In that case, the poster is uploaded to the cloud and
+    # the share link is attached to the communication
+    # TODO: potentially rethink the way images are attached to communications
+
+    visibility: bool
+    # should this communication be shown to members at all? Useful for
+    # temporarily hiding a communication
+    
+    only_members: bool
+    # is this communication only visible to members? This refers to visibility
+    # on the website, i.e. is this only visible to visitors who are logged in?
+    
+    archived: bool
+    # an archived communication cannot be edited, only viewed for future
+    # reference
+
+
+
+class ActivityModel(CommunicationModel):
+    category: Annotated[
+        CommunicationCategory,
+        Field(pattern=CommunicationCategory.activity)
+    ]
+
+    signuplist: GroupRef = GroupRef("signuplist", None)
+    # TODO: validate that a signuplist can only be attached if category is
+    # activity? Or should this not be validated?
+
+    start_time: datetime
+    end_time: datetime
+
+    @model_validator(mode='after')
+    def check_end_after_start(self) -> Self:
+        if self.end_time < self.start_time:
+            raise ValueError("End time cannot be earlier than start time")
+        return self
diff --git a/src/fiber_package/groups/signuplist.py b/src/fiber_package/groups/signuplist.py
new file mode 100644
index 0000000..042c2a4
--- /dev/null
+++ b/src/fiber_package/groups/signuplist.py
@@ -0,0 +1,12 @@
+from pydantic import BaseModel
+
+from fiber_package.util import GroupRef
+
+
+'''
+A sign-up list is a list of members that have signed up for a given activity.
+'''
+class SignuplistModel(BaseModel):
+    activity: GroupRef = GroupRef("communication", None)
+    # TODO: validate that the communication is indeed an activity (or should
+    # this be done?)
diff --git a/src/fiber_package/util.py b/src/fiber_package/util.py
index 8973966..6077523 100644
--- a/src/fiber_package/util.py
+++ b/src/fiber_package/util.py
@@ -15,9 +15,9 @@ class GroupList:
 @dataclass(slots=True)
 class EntityRef:
     template: str
-    uuids: UUID4 | None = None
+    uuid: UUID4 | None = None
 
 @dataclass(slots=True)
 class GroupRef:
     template: str
-    uuids: UUID4 | None = None
\ No newline at end of file
+    uuid: UUID4 | None = None