How can I get all groups for an specific app?

AFAIK, there is no direct function to get the admin group for a specific app.

Considering only application prefix is given, you can build a expression for this like below which would ofcourse depend on what naming convention you use for giving group names. 

a!localVariables(
  local!allGroups: getallgroupsnames(100, 1),
  reject(
    isnull(_),
    a!forEach(
      local!allGroups,
      if(
        like(
          fv!item,
          ri!applicationPrefix & "*" & "Admin*"
        ),
        fv!item,
        null
      )
    )
  )
)

But what is the need for it??