Opus Dei exists for formation. Membership exists (and types of membership exist) to handle the varying availability of different people, that is, in terms of how available they are to support the Work’s activities.
Yes, you can attend various and certain forms of formation w/out being a member.
There are collective and individual means of formation.
Collective means would include attending retreats, recollections, workshops, and circles.
Individual means would include spiritual direction and chats, etc.
Opus Dei priests are in the confessional a lot. They are very encouraging, and simple. Forthright. They are self-less and smiling and giving.
But membership is really a vocation that has to be discerned over a period of time.
The mortification point is a part of the spirituality of Opus Dei, but the cilice and discipline is reserved for numerary (unmarried) members and priests, done under the specific direction of their Spiritual director. These means of mortification have been part of the Church’s history (Mother Theresa, etc) for many many years. They are not inventions of Opus Dei.
But mortification should be a part of all our lives, every day. Smiling when we don’t want to was offered by St Josemaria Escriva as one of the best forms of mortification. We can pour a half a glass of wine, or none at all, get up 10 minutes earlier, with vigor. We can work 5 minutes longer, giving a bit more love and attention to our work. Not working in an open ended fashion (deadlines), etc.