In this context, "il est difficile à joindre" stands for "it's hard to get in touch with him". It's chiefly used as "get in touch by phone" Here are a few examples: - Je n'arrive pas à le joindre [au téléphone] - Il n'est pas facile à joindre [au téléphone] - N'hésitez pas à me joindre au téléphone. Voici mon numéro… More @Wikipedia
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