Anybody may be a Shali'ach to bring a Get with the exception of a deaf-mute, Shoteh and Katan.
If a Shali'ach gives a Get and becomes a deaf-mute before he has a chance to say b'Fanai Nichtav the signatures must be authenticated.
Rabah holds that even after the Din of Lishmah became public knowledge, it is not sufficient to authenticate the signatures and the Shali'ach must say b'Fanai Nichtav. (1)
A woman may be a Shali'ach from her husband to bring her Get and she must say b'Fanai Nichtav. (2)
If the husband brings the Get himself it is not necessary for him to say b'Fanai Nichtav.
If two Sheluchim bring a Get it is not necessary for them to say b'Fanai Nichtav.
If a Shali'ach brings a Get from Medinas ha'Yam and fails to say b'Fanai Nichtav if the signatures are authenticated the Get is Kosher. (3)
R. Yehoshua Ben Levi holds that a Shali'ach who brings a Get must say b'Fanai Nichtav because of the concern of Lishmah. (4)
It is a Machlokes Amora'im whether a Shali'ach must give the Get to the wife in front of two or three people.
For Edus d'Rabanan a witness can also be a judge.
R. Meir says if a Shali'ach brings a Get and fails to say b'Fanai Nichtav a child that is born after the woman remarries is a Mamzer. (5)
A BIT MORE
1. Because maybe people will eventually forget the Din that a Get must be written Lishmah.
2. If the husband gave her the Get as a Shali'ach and said that she will not be divorced until she brings the Get to Beis Din and they appoint a Shali'ach to give her the Get.
3. According to Rabah the Get is only Kosher b'Di'eved if she already got remarried on the basis of the Get.
4. R. Yochanan holds that the purpose of saying b'Fanai Nichtav is not for Lishmah but rather in order to authenticate the signatures.
5. R. Meir holds that anytime a Rabbinic ordinance is altered the child is a Mamzer.
If a Shali'ach brings a Get from Medinas ha'Yam and fails to say b'Fanai Nichtav, if the signatures are authenticated the Get is Kosher. If the Get is lost before the Get is authenticated the woman is Safek Megureshes (Rosh in the name of the Rambam). However she is only Safek Megureshes if the husband claims that the Get is forged, but if the husband does not make a claim she is regarded as definite Megureshes and if she remarries she may stay with her husband. (Tif'eres Shmuel)
WRITTEN AND SIGNED IN DIFFERENT PLACES
If a Get was written in Eretz Yisrael and signed in Chutz la'Aretz the Shali'ach must say b'Fanai Nichtav. If it was written in Chutz la'Aretz and signed in Eretz Yisrael it is not necessary for the Shali'ach to say b'Fanai Nichtav. (Rambam Hilchos Gerushin 7:11)