I know some Ashk'nazim say rabosay n'varech, some rabosay mir velen bentschen, and some hertzi rabosay mir velen bentschen. (Any other forms?) Is there any particular reason given for one or another?
See the excellent article here regarding the proper nussach of zimmun.
The highlights are: that the introductory bit (everything before Nevarech she'achalnu mishelo) is based on a ruling of the Zohar quoted in the Magen Avraham quoted in the Mishna Berura (OC 192 sk 2) and in the Aruch HaShulchan (OC 192:2) which says that every "davar shebikusha" needs a oral preparation, we introduce the zimmun with an introductory call to join together. The phrase the Zohar mentions (as quoted in the Be'er Heitiv OC 192 sk 1) is Hav Lan VeNivrich which is an allusion to the Talmud Pesachim 103a-b where the phrase is used to let people know that bentching was soon, NOT as part of the zimmun (as evidenced by the parallel phrase in the gemara: Hav Lan VeNishteh). The Be'er Heitiv and Mishna Berura there and the Aruch Hashulchan there all point out that the custom is to say the initial bit in the vernacular, hence: rabosai mir vellen bentchin. Those who say hav lan venivrich are using the Zohar/Gemara's original wording. Everyone else is using their own vernacular or some original combination/formulation of honorifics and blessing-verbs, as all we are looking for is an oral preparation. I have even see it done to "Ok Guys, Let's Bentch!"