When I was a young law firm associate back in the day, I was second chair on a complex, lucrative (for our client) and acrimonious technology/patent license negotiation. We were holed up in a conference room with buyer's counsel, in the dead of a typical brutal Colorado winter, groping our way towards final agreement. The legal negotiations were excruciating. We were the smaller party going up against a huge computer manufacturer, and the buyer's counsel, on the junior side of the experience scale, simply kept repeating the same mantra that corporate policy forbids any flexibility, even on what clearly should be uncontentious points.
My boss (senior partner at my firm) eventually lost his patience -- but never lost his cool. I can hear his words to this day:
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, and those teaching law in the legal academy, are more likely to be admitted.