。
How long have those guys been playing tennis?
Literal
Those-guys [topic-は] since-when tennis [object-を] are-doing [question-ん ですか].
いつから ('since when') + the continuous ~ている is the standard pattern for 'how long have you been (doing) ~' — literally 'since when are you doing ~.' The explanatory ~んですか softens the question into one that seeks background context, not just a raw fact. 連中 here is the colloquial collective pronoun ('those guys, that bunch') — dismissive or familiar depending on tone, never neutral.