Grass huts to sell from weren't the only marijuana related buildings that the hippies of Woodstock built - they also had larger projects on their mind. In this picture, we're looking at a 'smoke camp', a shelter that for all intents and purposes had the same function as a pub.
Smoke camps were designed to fit multiple people in and give them a communal space to bring their alcohol, share a joint, get to meet new people and socialize. The whole festival had an atmosphere of sharing and sociability, but it was sometimes nicer to take it all in a few people at a time rather than 400,000 people all together. People met in these smoke huts and formed friendships that lasted the rest of their lives. Others just popped in to hear an amusing anecdote before they set off to find the rest of their friends.