"Guild" versus "association"

Solution 1:

Obviously "correct" usage is whatever [the group in question's] founders originally decided to call themselves (there are also Federations, Fellowships, Societies, Unions, etc.)

Guild has a somewhat archaic flavour, but a newly-formed group might choose it simply to imply a long and distinguished history. It also has stronger overtones of the members being [traditional] craftsmen with tradeable skills and/or products.

Association may imply a looser (often predominantly commercial) relationship between members.

Solution 2:

Guild is the word used for "an association of people for mutual aid or the pursuit of a common goal." It is then a word that has a more specific meaning than association.

The other meanings of guild are "a medieval association of craftsmen or merchants, often having considerable power" and (in ecology) "a group of species that have similar requirements and play a similar role within a community." (Both of them don't apply in the specific case.)