Achdus - does “Achdus” mean I am supposed to embrace Jews I really don’t agree with?

Is Achdus a Halachic imperative, a Hashkafic concept or something else?
What’s the definition of “Achdus”? Is it different from “Kiruv”?
Can we outwardly express our disagreements?
Should we look to have Achdus with groups and movements of Jews who have values that are antithetical to Torah?