So do not do otherwise, but be knowledgeable with these young men and come hither to us as to your friends and very family.
[Translating this sentence is like disentangling your Christmas lights after you threw them in the box last year: it's not impossible to do, though it is difficult, and by the time you're done, some of the lights no longer work. It's possible to translate this sentence into passable English, but some aspect of the Greek is going to be left out. Mine leaves out the notion of resort in the verb phoita. It would have been strained to include it. Let me give some other translations (beginning with my least favorite and going on to better ones) before explaining mine.
Cornford: "So you must not disappoint me. Treat us like old friends, and come here often to have a talk with these young men." (This trans. Inexplicably moves the "old friends" before the "young men.")
Grube: "So be sure to come more often and talk to these youngsters, as you would to good friends and relations." (Ignores the first clause. Also uses the word "youngsters"; the youngsters in question are not children but young adult men.)
Griffith: "So do please spend some time with these young men. Do come here and visit us. Regard us as your friends -- as your family, even." (Takes three sentences to do what Plato does in one. Ignores the first clause, too.)
Lee: "So don't refuse me, but come and talk to the young men here and visit us as if we were old friends." (Omits "relatives" at the end.)
Reeve: "So do as I say: Stay with these young men now, but come regularly to see us, just as you would to friends or relatives."
Bloom: "Now do as I say: be with these young men, but come here regularly to us as to friends and your very own kin."
Shorey: "Don't refuse then, but be yourself a companion to these lads and make our house your resort and regard us as your very good friends and intimates."
Regarding the first clause, my translation is literal, but I don't think it's awkward, or at least awkward enough to require something else. In the second phrase, the awkwardness begins. The sticky wicket is sunisthi, the present imperative (2s) of sunoida, meaning "share in knowledge of" or "be cognizant of." This is what I'm translating as "be knowledgeable with." I'll let the reader judge whether this is better than the alternatives listed above. Most of them use a "talk" instead of a "know" verb, but I don't think "talk" is sufficient to convey the meaning of sunoida. But I admit that my translation is awkward. (I'm also wondering if there are any implications that knowing has connotations of "knowing" in the KJV, as in "Adam knew Eve.")
Reeve's translation also implies that S. will stay with the young men now and come back to see "us" (who? C. using the royal we?) later. The choice in the Greek is not clear. Can S. do both at once, or does he have to choose between being with the young men and being with C.? If so, why? Adam has some relevant comments. He seems to take the young men to be S.'s friends from Athens, and "us" as referring to C., P., and the other (foreigners?) living in the Piraeus. So the sense might be "S., associate with the young men of Athens, but come also to see us in the Piraeus."
I've added "your" before "friends." It seemed to make the sentence read better. Maybe it's unnecessary.
For the preceding sentences -- I know it's been a while -- see here and here.]