A validation rule on the Opportunity object is the best way to enforce the requirement that the Reason Lost field is required when the stage is changed to Closed Lost. A validation rule can specify a condition and an error message to display when the condition is not met. For example, the condition could be AND (ISPICKVAL(StageName, "Closed Lost"), ISBLANK(Reason_Lost__c)) and the error message could be
"Please enter a reason for losing the opportunity." Making the field required on the page layout or universally required would not work, because the field should only be required when the stage is changed to Closed Lost, not for other stages.

Explanation
The type of group that the associate should create to share records about a carpool program with users is a public group. A public group is a set of users that can be used to define sharing rules, queues, and other features that require a group of users. A public group can include individual users, other groups, roles, or territories. A public group can be used to share records with a specific group of users who are interested in the carpool program. A private group is a type of Chatter group that is not related to record sharing. A queue is a feature that allows users to manage leads, cases, and other objects as a team, but it is not suitable for sharing records about a carpool program.