In a discussion with erik_k of help.com it seems apparent that tag-based communities are one of the premier forms of ad-hoc community building.
Rather than a formal process to start a community, people create tags around what they are working on or are interested in and it matches the folksonomy of others with the same tags, and what they collectively post about it.
This form of tag-based community isn't unique to help.com; there are other sites like 43 Things, del.icio.us, jeteye.com, etc. that all allow people to "bubble up" a community based on what they tag.
If people use the same keywords, they automatically become a collective or group of participants around the topic. With sites like Jeteye, you can create multiple types of content elements in each tag. Still others allow member of a shared tag to start a discussion area, etc.
This kind of ad-hoc process allows a population to rapidly organize and create communities around what they are working on. You can build quite a sizeable community this way if you have a general purpose site like help.com or del.icio.us.
The downside is that it can be chaotic and hard to track. Some tags get buried under others; there can be many, many tags that refer to the same thing but are disjoint, simply based on how you word the tag; there may not be a central or high-level navigation system. Finally, it's hard to "value" what one person creates over another.
The alternative is to have a formal community creation process and then build community tools around the created entity. This also has its weak points which primarily center around the fact that the process can become bureaucratic.
This isn't limited to just information, but also to other scenarios such as situational applications, where people get together to quickly prototype or build an application in a group setting. More on that later.
Tag-based ad-hoc communities