WASHINGTON — Representative Anthony D. Weiner has told friends that he plans to resign his seat after coming under growing pressure from his Democratic colleagues to leave the House in the wake of revelations of his lewd online exchanges with women, said a person told of Mr. Weiner’s plans. The news comes as Democratic leaders prepared to hold a meeting on Thursday to discuss whether to strip the 46-year-old Congressman of his committee assignments, a blow which would severely damage his effectiveness.