There is a lot to be said for having your users register for the site, however, this can decrease participation. By registering, you have some way to know who is doing what and to directly deal with the challenging people. Without that you might not know that the three actions all came from one troublesome person. Legally there is some merit to having someone physically check a box that agrees to play by the rules. Registration can be as simple as a name and email address or as complicated as a long questionnaire. Studies have shown that if you go too long, then the dropout rate climbs rapidly.
Building a well designed registration can ensure that the maximum number of people make it through the process to contribute to the community. There are several theories of when and how to position registration. Generally you would prefer to position the registration or joining after you have the chance to show the value proposition of the community. This helps the user balance the value they get for giving you the information. If you can show some content and then ask for registration for more detailed content, that works as well. Having a distinct additional value for the act of registering is critical in the “sell” of joining a community.
In the process of registration… take the time to let them know why you want the information and what you will and will not use it for. For example, in a registration someone ask for your address. If there is no context people will assume the worst. But let’s say that you send members that get to 100 posts a t-shirt. If you share the why, then they will be much more likely to give you the information.
You should also consider the concept of graduated registration. Start by asking for only the basic information like say email; username; password. This makes it simple to get someone in the door and closer to that first all critical interaction. After you have engaged them further, perhaps you go back and ask them for more of the detailed information you need? (be sure to tell them why)
There are very few instances where I recommend open participation. One example recently was when I was working with a brand where the audience was an elderly crowd who would not be comfortable within the internet space. I felt like the potential loss of participation within the registration process would be enough to consider open participation for this particular community. When I do recommend it, I always pair it with pre-moderation of the content. This allows you to catch bots and people that are just being ugly before the content goes live. In addition, this pre-moderation, should be considered as an additional time commitment for the moderators.