The 2nd sentence is grammatically correct, but it is not natural. "Easily" is an adverb, which means it describes the verb.
So, in the first sentence, you cannot use "easily" because you need an adjective with the linking verb "is."
"Let's go back to the old place because it will be easy for them to find us."
(I would further change this to be future tense (to "will be" because the speaker is talking about something that is not happening yet.)
The 2nd sentence is grammatical, but it is in the passive voice meaning that the subject of the sentence is not the one performing an action. This is not frequently used by native speakers. You might want to put it into active voice.
"Let's go back to the old place because they can easily find that place."
(I also changed "this" to "that" because the place is not where the speaker is yet.)