I assume you mean if there's a multiplayer game, and you are not the active players.
If you are using the attack multiple players option for a multiplayer game, each player can only block the creatures attacking him or a planeswalker he controls.
802.4a A defending player can block only with creatures he or she controls. Those creatures can block only creatures attacking that player or a planeswalker that player controls.
I'm not quite sure what you're asking. Let's look at Predatory Rampage :
Each creature your opponents control blocks this turn if able.A creature is only able to block an attacking creature if it is controlled by the defending player. If there is a three-player game with Players A, B, and C and A attacks B, only player B's creatures may block the attackers. Players A and C are not able to block: A is the attacking player and neither is the defending player.
It depends on how you and your friends set up your rules, sometimes me and my friends like to bend a few things like in a FFA we are allowed to block for other people resulting in a more diplomatic type of game where people help others when needed then backstab each other.
You declare which opponent(s) you're attacking when you declare attackers. If you swing with everything at one opponent after casting Predatory Rampage , that's the only player who can block.
In free-for-all formats, players can't normally block for other players. The only way Predatory Rampage would force an opponent to block while attacking is if you attack that opponent.