In US: must be an attorney licensed and in good standing in any state, territory or DC.
Outside US: must be a lawyer or equivalent (eg counselor, barrister, advocate, solicitor), duly educated and licensed/accredited and in good standing.
As a general rule, experienced and currently practicing lawyers are more likely to be admitted.
It is the rare lawyer that takes a keen interest in a topic to the point of research and publication. Even rarer is the lawyer who courts criticism and reaction. By teaching, and accepting challenge, such a lawyer becomes an arena lawyer, willing to put herself out there.
The arena lawyer becomes an expert, and in doing so, adds to the collective knowledge of our trade.
And experts—even lawyers—are experts first, and competitors second. Law firms large and small go to great lengths to source original content, as a means of demonstrating expertise. This expertise can be easily appropriated by the competition, and yet the freely shared legal content flow never ceases.
On the origins of "Esquire" (or "Esq.") as an honorific for lawyers:
The word itself derives from Old French, and in turn from Latin, where it means something like “shield-holder.” In the 1200s and 1300s in England, a variety of languages were used, so such figures might be referred to as the … French escuier, which became “esquire.” These terms all refer to roughly similar people. This role was generally considered moderately prestigious for young men of some wealth, but at its core it was a service job. You carry a knight’s stuff, tend to his horses, that kind of thing. “Esquire” and “squire” were names for the same gig for a few hundred years.