My sister knows.
We are close but don't see each other much. She knows what I have been through and how much I am suffering. One day I choked up a bit when she simply asked me how I was doing she asked me bluntly if I had plans to hurt myself. It was so obvious it was not really a question but a way to tell me she knew about my intentions and an invitation to talk if I wanted to. I love my sister; I cannot lie to her. So I told her.
I told her about my plans, not in every detail, but she understood I was extremely determined and well advanced in my preparations. In the following weeks we had several open and very intense discussions, and she was incredibly respectful. In the end she told me it made her sad, she did not want me to go, but that she would support me entirely whatever my decision. I know it is a incredible act of love to be able to accept my suicide, without judgment, without putting her fears before my suffering.
A friend of mine also knows. She, too, asked me bluntly one day. I didn't hide I had suicidal thoughts, but I didn't want to scare her and put her in a difficult situation, so we talk more about life than death. She knows when to nudge me, gives me a wink and a smile, and drags me out when I feel low. I know she is there for me.
Many of my other friends are certainly aware too, but they don't ask so I don't tell. I believe they prefer not to know and would be uncomfortable if suicide came up in the conversation. They wouldn't know how to react and it would make our relation awkward. So we tacitly behave 'normally' and that's ok. I expect they will be only half-surprised when I will ctb.