Cooperative Variant
The rulebook that ships with Portals and Prophets does contain a number of rule variations to accommodate special gameplay situations. For example, there is a variant that simplifies the rules to make it easier for young players to participate. There is also a variant that allows for team competition. However the rulebook does not contain rules for all players playing cooperatively.
The good news is you can download a separate rulebook for the cooperative variant of Portals and Prophets that uses all the same game components already included in the game. In the cooperative variant, the time machine is failing and players must work together to repair the time machine while simultaneously attempting to visit all the events they want to see. Teamwork and strategic planning are required to be successful.
The rules describe how to adjust the difficulty of the challenge so the difficulty of the challenge matches your collective skill/experience level.