diff --git a/lib/permissions/minimally-encoded-thread-permissions.js b/lib/permissions/minimally-encoded-thread-permissions.js
--- a/lib/permissions/minimally-encoded-thread-permissions.js
+++ b/lib/permissions/minimally-encoded-thread-permissions.js
@@ -133,10 +133,15 @@
   'toplevel_': BigInt(2),
   'opentoplevel_': BigInt(3),
 });
+const membershipPrefixes = Object.freeze({
+  '': BigInt(0),
+  'member_': BigInt(1),
+});
 
 // Role Permission Bitmask Structure
-// [9 8 7 6 5 4 3 2 1 0] - bit positions
-// [b b b b b b p p f f] - symbol representation
+// [10 9 8 7 6 5 4 3 2 1 0] - bit positions
+// [m  b b b b b b p p f f] - symbol representation
+// m = membershipPrefix  (1 bit)
 // b = basePermission    (6 bits)
 // p = propagationPrefix (2 bits)
 // f = filterPrefix      (2 bits)
@@ -148,8 +153,11 @@
     propagationPrefixes[parsed.propagationPrefix ?? ''] & BigInt(3);
   const filterPrefixBits =
     filterPrefixes[parsed.filterPrefix ?? ''] & BigInt(3);
+  const membershipPrefixesBits =
+    membershipPrefixes[parsed.membershipPrefix ?? ''] & BigInt(1);
 
   const bitmask =
+    (membershipPrefixesBits << BigInt(10)) |
     (basePermissionBits << BigInt(4)) |
     (propagationPrefixBits << BigInt(2)) |
     filterPrefixBits;
@@ -167,18 +175,24 @@
 const inverseFilterPrefixes: Map<bigint, string> =
   invertObjectToMap(filterPrefixes);
 
+const inverseMembershipPrefixes: Map<bigint, string> =
+  invertObjectToMap(membershipPrefixes);
+
 const tHexEncodedRolePermission: TRegex = tRegex(/^[0-9a-fA-F]{3,}$/);
 const decodeRolePermissionBitmask = (bitmask: string): string => {
   const bitmaskInt = BigInt(`0x${bitmask}`);
   const basePermission = (bitmaskInt >> BigInt(4)) & BigInt(63);
   const propagationPrefix = (bitmaskInt >> BigInt(2)) & BigInt(3);
   const filterPrefix = bitmaskInt & BigInt(3);
+  const membershipPrefix = (bitmaskInt >> BigInt(10)) & BigInt(1);
 
   const basePermissionString =
     inverseBaseRolePermissionEncoding.get(basePermission);
   const propagationPrefixString =
     inversePropagationPrefixes.get(propagationPrefix) ?? '';
   const filterPrefixString = inverseFilterPrefixes.get(filterPrefix) ?? '';
+  const membershipPrefixString =
+    inverseMembershipPrefixes.get(membershipPrefix) ?? '';
 
   invariant(
     basePermissionString !== null &&
@@ -186,11 +200,18 @@
       propagationPrefixString !== null &&
       propagationPrefixString !== undefined &&
       filterPrefixString !== null &&
-      filterPrefixString !== undefined,
+      filterPrefixString !== undefined &&
+      membershipPrefixString !== null &&
+      membershipPrefixString !== undefined,
     'invalid bitmask',
   );
 
-  return `${propagationPrefixString}${filterPrefixString}${basePermissionString}`;
+  return (
+    propagationPrefixString +
+    filterPrefixString +
+    basePermissionString +
+    membershipPrefixString
+  );
 };
 
 const threadRolePermissionsBlobToBitmaskArray = (