Hide the join group button

Until it actually does something

src/skins/vector/css/matrix-react-sdk/structures/_MyGroups.scss

@@ -34,6 +34,11 @@ limitations under the License.
     width: 40%;
 }
 
+/* Until the button is wired up */
+.mx_MyGroups_joinBox {
+    visibility: hidden;
+}
+
 .mx_MyGroups_joinCreateHeader {
     font-weight: bold;
     margin-bottom: 10px;