。
Let's do a group date with the tennis club.
Literal
Tennis-club [with-と] group-date [do-let's-しましょう].
合コン is a contraction of 合同コンパ 'joint gathering / mixer' — a staple social institution of Japanese student and young-adult life, where a group of guys and a group of girls (often from different schools or clubs) meet up. と marks the other party (the tennis club). ~ましょう is the polite volitional 'let's.'