Is there a japanese equivalent to the english expression 'ENJOY'?
Friends invite you to go out to eat in a restaurant, but you can't come. So you say 'enjoy!'
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There is the verb 楽【たの】しむ, which could be translated as "to enjoy".
I often hear something similar to 「楽しんでください！」 in similar situations, which is simply an imperative form of the aforementioned verb. Among friends, you might say 「楽しんでね！」.
However, as macraf added, if you were invited to something and could not make it, by friends or not, a more polite expression would perhaps be more appropriate. For example, you could say something like 「ディナー／お時間を楽しんでお過ごしください。」.
Friends invite you to go out to eat in a restaurant, but you can't come, then you'd say 楽しんできてね！ (Lit. Enjoy and come back.)
I think I would probably just say 行ってらっしゃい though...