we are not using membrane, but LDAP users. We do cache the whole acl_users folder for speed. However everytime the users are changed, we have to invalidate memcached accordingly and have had to build a custom step to do this.
Also you need to be sure to not have any custom objects in your users that can't be converted to memcached's dicts.
Mind we are using pas.plugins.ldap and the invalidate is actually clearing the whole cache, because we can currently not get hands on all affected objects efficiently. Therefore we use a dedicated cache for the user management.