Is referring to the group “Jehovah’s Witnesses” an issue of saying Shem Havaya? Please explain.
Yes it is a serious issue of saying H-shem’s Holy name, and it should not be said. Saying H-shem’s name (Adon…) is not allowed, except when davening or learning. However the special name of H-V- H-A, is a name that we are never allowed to say. This name has no heterim except the kohen Gadol in the Bais Hamikdash on Yom Kippur. This is a serious sin as the Gemorah Sanhedrim 90a says that a person that says this name forfeits his place in the world to come! (A person however can do teshuva for any aveiro).
Sanhedrin 90a,Tosefos Avodah Zara 18a D:H Hoga, Rosh Yuma 8-19, Chayei Adam 5-27, M:B 5-2. Numerous poskim including R’ Moshe Reich shlit”a, author of Kodesh Lahashem.
It should be noted that although there is discussion regarding saying the name of an avodah zara when you are not referring to the avdah zara itself, such as when saying the name of a city of street named after an avodah zara. The poskim say it is permitted, however it is preferred to say the name in a different way, (see Mishna Halachos 9-169, and Rivivos Efrayim 8-556). Similarly it is permitted to say Lord Jakobson or Elohim Acheirim, because you are not referring to H-shem there. In our case, the name itself says that it is referring to H-shem, therefore it is not permitted. (Heard from Horav Shmuel Felder shlit”a).