In the case of cultural evolution, such "free riders" can often be identified and penalized. A green beard is a form of signalling. Here we will consider a uniform to be a similar form of cultural signalling. If you put on a nurse's uniform and visit a hospital you will probably be found out - if you engage in very many interactions with other staff members. Much the same would happen with an imitation police man in a police station or a fake soldier in the army. The uniform is only one of a large number of cultural traits marking out genuine members of these "tribes" - and it is difficult for an invader to fake all of the required markers. Some tribes develop marks of group membership that are even harder to forge - with piercings, tattoos, bizarre haircuts as well as distinctive clothing, habits and dialects. So: in the cultural realm, cheaters tend to get found out and punished - and that is one way in which the effect can be made to work.
The ideas in the field of tag-based cooperation are a little bit like those associated with the "green beard" effect. Studies of tag-based cooperation have shown that green beards can be less vulnerable to exploitation than was originally thought. For one thing, when a tag or marker is successfully exploited, another tag can be adopted. Also, the whole idea of having genes for altruism towards those with the 'green beards' had always seemed a little bit contrived. Fortunately, this turns out not to be necessary. In cultural evolution, organisms can simply learn which tags best signal altruism in their environment - and preferentially adopt them.
Cultural "tags" or "tribal markers" probably play a number of roles. Knowing who is in your tribe facilitates reciprocal altruism. It indicates who can be punished for defections against group members - and who future interactions can be expected with. Tags also facilitate cooperation based on cultural kin selection. If memes are able to credibly signal their presence in humans, related memes may be able to use the perceptions of their hosts to identify copies of themselves in other people, allowing them to manipulate their hosts to act so as to favour copies of themselves in other bodies. Even without such behavioural manipulation, tags can still facilitate altruism - by allowing groups of cooperators to form and help each other.
The green beard effect has sometimes been used as an alternative explanation to kin selection. Mark Pagel uses the "green beard" terminology - instead of talking about cultural kin selection - in his book Wired for Culture. However it is important to remember that the green beard effect is a type of kin selection. Green beards indicate shared ancestry - and that is still true regardless of whether they are transmitted via DNA genes or culturally. In one case, genetic ancestry is involved. In the other case it is memetic ancestry.