I guess the idea is a bulletin board, originally: to post a note, as one does with paper on a bulletin board. "Sign" in Latin could be nota or index, and there are corresponding verbs notare and indicare.
To affix something is suffigere, and to place, collocare. Adstruere can to be build on top of, to add on, to contribute, and is also plausible, though perhaps not quite as germane.
To start a thread: filum or colloquium or sermonem incipere.
It seems we have a wealth of options.