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.
24 Oct 17
This New York Times story of the death of an intellectual property practitioner at Silicon Valley's Wilson Sonsini Goodrich & Rosati is both harrowing and heart-breaking. "He had been working more than 60 hours a week for 20 years, ever since he started law school and worked his way into a partnership …", only to slowly but inexorably become an addict of various drugs, finally felled by a bacterial infection common among IV drug users. The last call on his phone he ever made was to call into a conference for a client, mere hours before he succumbed.