The answer is somewhat complicated
Guinan was on Earth in the 19th century. Data travelled back in time from the 24th century to the 19th century. He saw an article in a newspaper about Guinan, and assumed that Guinan from the 24th century had travelled back in time to find him. In the 24th century Guinan told Picard to go back in time with the away team to find Data. When Picard arrived in the 19th century he met 19th century Guinan. So, in a sense, Guinan met Picard (of the 24th century) in the late 1800s (her species live verrrry long lives).
It's not clear when Picard first met Guinan from his perspective, though its established that they have known each other for many years.