When a Jehovah's Witness is reinstated how does the congregation react? When a disfellowshipped person takes the steps necessary to be accepted back into the Jehovah's Witness faith an announcement is made to the congregation informing all of the reinstatement of this person into the fold.

How is such an announcement worded and how is the congregation supposed to react to the news?
Beside baptism and marriage, reinstatement is one of the most joyous occasions Jehovah's Witnesses have. It feels very much like it did for the father of the prodigal son at Luke 15:17-24, as well as for the shepherd who found his lost sheep at Luke 15:4-7.
All announcements are supposed to be made in the last 3 minutes of a meeting. The reinstatement announcement is made to the congregation they were disfellowshipped from, as well as to the congregation they are currently in (if they've since moved). The announcement is basically guaranteed to be met with applause and is worded as:

[Name of person] is reinstated as one of Jehovah's Witnesses.

There are some relatively new publications for ones who desire to return to the congregation. There is the brochure, Return to Jehovah, as well as a modern-day drama, The Prodigal Returns.
The brochure contains this box on the last few pages:

How Will The Congregation Receive Me?
You can be sure that members of
the congregation will warmly welcome you. Rather than criticize or
judge you, they will confirm their love for you and do whatever they
can to encourage you.—Hebrews 10:24, 25.
“I was embarrassed to return to the Kingdom Hall. I wondered how the
friends would treat me. One of our elderly sisters who was there 30
years ago said to me, ‘Welcome home, Son!’ That really touched my
heart. I was truly home.”—Javier.
“I went to the Kingdom Hall and sat
in the last row so that no one would notice me. However, many began to
recognize me from the days when I had attended as a child. They
welcomed me and embraced me so affectionately that I felt an
overwhelming peace. It was as if I had come home.”—Marco.