Ah, the need for speculation once again raises its ugly head. According to the letter of what we hear her say, they didn't meet face to face until she showed up on the Enterprise, yet they have known each other for years.
It's possible Guinan and Picard corresponded for many years before he asked her to join him aboard the Enterprise. She could have used any excuse to begin the correspondence, even saying she had heard about his exploits and wanted the straight deal directly from him. However their ongoing conversation in print might have gone, if they began sharing private details of their lives, the dialog between them becomes one "beyond friendship and family", as these details are ones they have not shared with others. It says a lot about Picard's trust of Guinan that he would ask her to come among his friends and co-workers, after sharing such intimacies with her.
Once again, this is all just non-canon speculation. But that's my take on it.