There doesn't seem to be much information about this. There are two relevant entries in the"The Tale of YearsYears":
- TA1300: ... The NazgulNazgûl reappear. The Chief of these comes north to Angmar.
- TA1980: The Witch-king comes to Mordor and there gathers the NazgulNazgûl.
The war between Arthedain and Angmar is described in some detail in LotR appendixThe Lord of the Rings, Appendix A, parts (iii) and (iv), but I can't see any mention of the other eight NazgulNazgûl. Therefore I think we can conclude that they were not involved here. We might speculate that they were active elsewhere, stirring up other enemies of the West (e.g. the Wainriders, who began their attacks on Gondor in TA1851). The entry for NazgulNazgûl in the Complete Guide to Middle-Earth The Complete Guide to Middle-earth (usually a reliable source) says that:
The other eight remained in the East until about 1640, when they secretly entered Mordor and began to prepare that realm for Sauron...
This makes sense (the Great Plague struck in TA1636, weakening the watch on Mordor), but I can't find canon evidence to support it.