Hi Clarissa,
We have two options to achieve it:
1. Using Selection via Sets
In above screen shot, if you see "selection via Sets - Site" (I have defined a set name (Test) and under this set, I have maintained the Site(s) to be included. For each user define different set. User before executing has to choose his set.
2, Using Variant
For each user define a unique variant (probably with their user id), each variant will have a unique BE.
This way you can achieve your requirement.
Regards,
Amar