A few years ago, the ABA presented a panel discussion, When Good Lawyers Make Bad Decisions, moderated by Serina Vash, a former prosecutor in the US Attorney's Office in New Jersey.
The panelists consisted of an acting FBI agent and three formerly-licensed lawyers, all of whom pled guilty to felonies. Each of these ex-lawyers had not only impeccable pedigrees (Princeton, Columbia, Univ. VA/London School of Economics), but were highly successful in their respective law practices—at least, up until the time the troubles for each of them began.
And their troubles began, tragically, as the consequence of enormous pressures in their personal lives. The discussion was mesmerizing. Each lawyer patiently and in great detail recounted his or her slow descent into ethical lapses and, eventually, criminality.
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 in the legal academy, are more likely to be admitted.