They get excited about their inside jokes. この表現を使えば、「内輪ネタで盛り上がる」とうまく伝えることができます。
他の表現としては、
They only care about their inside jokes.
They're always stuck on their private jokes.
例文としては、
It's hard to join their conversation because they get excited about their inside jokes. (彼らの内輪ネタで盛り上がるから、会話に入りにくい。)
They make it difficult for others because they're always stuck on their private jokes. (彼らはいつも内輪ネタで盛り上がるから、他の人が入りにくい。)