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.
In a study published in the Journal of Corporation Law, law school Professors Badawi and Webber examined, over a five-year period, takeover target share price changes in reaction to the perceived quality of the law firm(s) filing litigation challenging, on behalf of institutional shareholders, the fairness of the announced proposed merger or acquisition. The professors grouped the plaintiff firms into "high quality" and "low quality", based on a number of critieria, including value of settlements recovered and whether any of the firms were openly criticized by the Delaware Chancery judges as being, well, worthless leeches, basically.