The tense you want to use in these two sentences is the passé composé.
It's a compound tense. It needs an auxiliary (être or avoir) and the past participle of the verb.
Please read those :
- A beginner's lesson on the passé composé in French.
- A beginner's lesson on the passé composé in English.
You can see from the lessons that rentrer (derived from entrer) needs être as an auxiliary. And retrouver (a transitive verb) needs avoir.
The choice of être or avoir has nothing to do with what some English grammars call "continuous". This continuous aspect does not exist in French, forget about it when you study French.
Whether you use être or avoir does not change the tense of the sentence.
Although I'm all against translating when learning a language, I'll dot it here to hammer home the point. Both sentences would translate either into the preterite in English or the present perfect according to context.
Je les ai vus, ils sont rentrés en ville hier.
That would be a preterite in English.
Leur fille est tombée malade et ils sont rentrés en ville.
That would be a present perfect in English.
Rentrer and retrouver have different meanings and of course you could not use one in place of the other if you want your sentences to keep their meanings.