diff --git a/amanuensis/backend/membership.py b/amanuensis/backend/membership.py
index 7440353..5e9af13 100644
--- a/amanuensis/backend/membership.py
+++ b/amanuensis/backend/membership.py
@@ -5,6 +5,7 @@ Membership query interface
 from sqlalchemy import select, func
 
 from amanuensis.db import DbContext, Membership
+from amanuensis.db.models import Lexicon
 from amanuensis.errors import ArgumentError
 
 
@@ -36,6 +37,25 @@ def create(
     ):
         raise ArgumentError("User is already a member of lexicon")
 
+    # get reference to lexicon for next few checks
+    lex: Lexicon = db(
+        select(Lexicon).where(Lexicon.id == lexicon_id)
+    ).scalar_one_or_none()
+    if not lex:
+        raise ArgumentError("could not find lexicon")
+
+    # Verify lexicon is joinable
+    if not lex.joinable:
+        raise ArgumentError("Can't join: Lexicon is not joinable")
+
+    # Verify lexicon is not full
+    if lex.player_limit is not None:
+        if (
+            db(select(func.count()).where(Membership.lexicon_id == lexicon_id)).scalar()
+            >= lex.player_limit
+        ):
+            raise ArgumentError("Can't join: Lexicon is full")
+
     new_membership = Membership(
         user_id=user_id,
         lexicon_id=lexicon_id,
diff --git a/amanuensis/backend/post.py b/amanuensis/backend/post.py
index 3544b90..0a6373f 100644
--- a/amanuensis/backend/post.py
+++ b/amanuensis/backend/post.py
@@ -4,9 +4,10 @@ Post query interface
 
 import re
 
-from sqlalchemy import select, func
+from sqlalchemy import select
 
 from amanuensis.db import DbContext, Post
+from amanuensis.db.models import Lexicon
 from amanuensis.errors import ArgumentError
 
 
@@ -34,6 +35,14 @@ def create(
     if not body.strip():
         raise ArgumentError("Post body cannot be empty.")
 
+    # Check that the lexicon allows posting
+    if not (
+        db(select(Lexicon).where(Lexicon.id == lexicon_id))
+        .scalar_one_or_none()
+        .allow_post
+    ):
+        raise ArgumentError("Lexicon does not allow posting.")
+
     new_post = Post(lexicon_id=lexicon_id, user_id=user_id, body=body)
     db.session.add(new_post)
     db.session.commit()
diff --git a/tests/conftest.py b/tests/conftest.py
index a93b93c..5bd0b03 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -49,7 +49,10 @@ def make_lexicon(db: DbContext):
         }
         state["nonce"] += 1
         updated_kwargs = {**default_kwargs, **kwargs}
-        return lexiq.create(db, **updated_kwargs)
+        lex = lexiq.create(db, **updated_kwargs)
+        lex.joinable = True
+        db.session.commit()
+        return lex
 
     return lexicon_factory
 
diff --git a/tests/test_membership.py b/tests/test_membership.py
index 45e70a1..d6a0637 100644
--- a/tests/test_membership.py
+++ b/tests/test_membership.py
@@ -15,7 +15,15 @@ def test_create_membership(db: DbContext, make):
     new_lexicon: Lexicon = make.lexicon()
     assert new_lexicon.id, "Failed to create lexicon"
 
-    # Add the user to the lexicon as an editor
+    # Joining doesn't work when joinable is false
+    new_lexicon.joinable = False
+    db.session.commit()
+    with pytest.raises(ArgumentError):
+        memq.create(db, new_user.id, new_lexicon.id, True)
+
+    # Joining works when joinable is true
+    new_lexicon.joinable = True
+    db.session.commit()
     mem: Membership = memq.create(db, new_user.id, new_lexicon.id, True)
     assert mem, "Failed to create membership"
 
@@ -38,3 +46,16 @@ def test_create_membership(db: DbContext, make):
     # Check that joining twice is not allowed
     with pytest.raises(ArgumentError):
         memq.create(db, new_user.id, new_lexicon.id, False)
+
+    # Check that joining full lexicon not allowed
+    new_lexicon.player_limit = 1
+    db.session.commit()
+    two_user: User = make.user()
+
+    with pytest.raises(ArgumentError):
+        memq.create(db, two_user.id, new_lexicon.id, False)
+
+    new_lexicon.player_limit = 2
+    db.session.commit()
+    mem2: Membership = memq.create(db, two_user.id, new_lexicon.id, False)
+    assert mem2, "Failed to join lexicon with open player slots"