JOEL - Speaking of those in the Telestial world we read in the D&C:

"There are they who receive not of his fulness in the eternal world, but of the Holy Spirit through the ministration of the terrestrial;
And the terrestrial through the ministration of the celestial." (D&C 76:86-87)

According to Apostle Alvin R. Dyer:
"Thus we are led to understand, to some extent at least, the powers of dominion. The telestial receives its administration from the terrestrial where the presence of the Son will govern and under whose direction the priesthood after the order of the Son will administer unto this kingdom, as well as also the administering angels from the celestial kingdom. John the Apostle is an administering apostle unto the heirs of salvation. These of the celestial kingdom as referred to, no doubt directed by the Christ, will administer and govern the lesser glories. In all of this we see a purpose. Every one of us dwelt in the pre-mortal estate with God and was given challenge of leadership, and privileged to come to earth at this particular time to fulfil that challenge and prove worthy of further leadership appointment in the continuing plan of the gospel." ("Meaning of Truth" Alvin R. Dyer)

President Joseph Fielding Smith said:
"For we read also here in this vision, where the glories are spoken of, that those who dwell in the celestial kingdom shall minister unto those of the terrestrial kingdom; those in the terrestrial kingdom shall minister to those of the telestial kingdom.
The Son may go to the terrestrial, but they who enter into that kingdom shall not receive the fulness of the Father; they will not see the greatness of his glory. He withholds that from them. They never come back again into the fulness of his presence. Those who enter into the telestial kingdom will not receive the fulness of the Father or of the Son. They will not visit there but will send messengers to visit the inhabitants of that glory and instruct them.
Those in the terrestrial kingdom shall visit those in the telestial kingdom, and those of the celestial shall visit those in the terrestrial kingdom. Where the Father is these cannot come, for the Lord has said: "Where God and Christ dwell they cannot come, worlds without end. (D&C 76:112)" (Joseph Fielding Smith, Doctrines of Salvation, Volume 2, p.5)

And Apostle Melvin J. Ballard said:
"I have several times been asked, how is it possible for those who attain celestial glory to ever feel happy and satisfied to know that their children are in the telestial world, and never would have the privilege of coming up with their parents in the celestial kingdom.
We must not overlook the fact that those who attain to the higher glories may minister unto and visit and associate with those of the lesser kingdoms. While the lesser may not come up, they may still enjoy the companionship of their loved ones who are in higher station. Also we must not forget that even the least degree of glory, as the Lord has expressed it, is beyond all our present understanding. So that they are in the presence of glorious conditions, even though they attain unto the least place," ("The Degrees of Glory" A Discourse by Melvin J. Ballard, Delivered in the Ogden Tabernacle, September 22, 1922)

So according to the scriptures and these statements, those in the Celestial Kingdom may visit and minister to those in the Terrestrial world and those in the Terrestrial word may visit those in the Telestial world. The D&C scriptures seem to be saying however, that those in the Celestial world would not visit the Telestial world, although a couple of the statements above seems to say that they can. Joseph Smith never elaborated on these particular scriptures so it's hard to know exactly what is possible.