I don'tuse a special tag. The rul I use (for parent child build), is to have nothing checked (i.e. allow moves unchecked, allow property changes unchecked, donnotshare unchecked etc). member update I select merge.
By default a parent/child file will build the rollup with the first instance as the stored member then secondary instances as shared members. If it is not happening for you, I recommend looking at the dimension build properties in the load rule. I'll guess move shared members is checked. It you do not have a parent/child file then you can still build shared rollups. For example suppose you havea level based file and level 0 should be shared, then your file would have to look like
lev0, lev1,lev2,lev3,Lev1 (the shared parent), lev2
You can check here if you are satisfying the conditions for creating shared members under the Hierarchy (refer to Alternate Hierarchies). Plz note that rules for shared members are different in ASO world.