“Вы можете назначать” -> “назначьте” to save some space. The administrator may remain a subscriber, so “снять полномочия” seems a better option than “удалить”. “Нажмите и удерживайте” is too vague, at least in Russian (What do you need to tap and hold?). There is a menu symbol next to the name; tapping it does the same as tapping and holding the name, so I would extend the description by adding “или нажмите на значок меню (справа)” (if it fits).