You can use here("../") to access folders and files outside your project folder (i.e., project's working directory).
For example, you can put the shared data set in the folder named shared_data. Then read/import the data using read_csv(here("../shared_date/my_data.csv")).
The file structure may like this:
.
+-- project_01
| +-- result
| \-- scripts
+-- project_02
| +-- result
| \-- scripts
+-- project_03
| +-- result
| \-- scripts
\-- shared_data
\-- my_data.csv