In your case, you would use the in the second example:
For a number of years, the IAFC has convened a roundtable ...
The StackExchange question "Using the definite article with acronyms and initialisms" draws a distinction between acronyms (which are pronounced and usually don't take articles) and initialisms (which are not pronounced and generally take articles). IAFC is an initialism that is spelled out, and so the above advice suggests that one would use the with it. (I almost considered this a duplicate question for that reason.)
However, how the group refers to itself may vary, because exceptions to these rules exist and context is an interfering factor. (See ESA.) A look at the IAFC website highlights the contextual factors involved in using an article:
"About IAFC." A navigational menu item omits the article
"The IAFC Strategic Direction is a dynamic document serving as the foundation for the development and implementation of IAFC programs and services." Both usages depend on the noun phrase being modified by IAFC: "The ... Direction" requires the article because it refers to a specific, definite entity, but "programs and services" does not. This corresponds to your first example: "The ... Headquarters."
"Since 1873, the IAFC has provided a forum for fire and emergency service leaders ..." Used alone and as a noun, IAFC takes the. This corresponds to your second example.
In short, check body text on a website, consider whether the examples you are looking at use the abbreviation or initialism as a noun or adjective, and then adapt your writing style accordingly.