Beat Up inflicts damage on the target from each conscious Pokémon in the user's party which does not have a non-volatile status.
Source: Bulbapedia/ Beat Up
It means they are not:
- Burn
- Freeze
- Paralysis
- Poison
- Bad poison
- Sleep
If any Pokemon has any of these statuses, they will be skipped.
Check link in the paragraph above for more info.