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.
Often one party to a contract is allowed to act only with the consent of the other party, "not to be unreasonably withheld."
A common context in this regard is the right to assign an agreement. The language used is typically, "[Party A] may not assign this Agreement without the consent of [Party B], such consent not to be unreasonably withheld."
So what happens if Party B unreasonably withholds consent? Does the refusal then automagically become consent, such that Party A is free to assign away?
Jackie Ruffin, First woman lawyer in California championed the idea of public defenders (review of Woman Lawyer – The Trials of Clara Foltz by Barbara Babcock), 98 Women Lawyers Journal 38 (2013):
The life of Clara Foltz, 19th-century trailblazing lawyer, women’s rights activist and social reformer, has few equals for drama, achievement and sheer chutzpah. In Woman Lawyer, Professor Emerita Barbara Babcock, herself a trailblazer as the first woman appointed to the regular faculty at Stanford Law School, has written a richly detailed account of Foltz’s bold and remarkable life, replete with "firsts," including first female lawyer in California, first woman to serve in a statewide California office and first female deputy prosecutor.