Before pre-sale, some ICO also offers a private sale that is held privately prior to launching their ICO publically to gather investors they can be their own team/company members, friends, business partners or others. you cannot contribute in private sale.
Regarding your question, in pre-sale anyone can take part you can also take part because both pre-sale, main sale some also offers post-sale all are announced to the public. In pre-sale ICO usually, not all, the minimum investment can be very high but it will give you a high bonus if invested in pre-sale. In the main sale, the bonus will be less or no bonus.