I've been looking through the tutorials and the forum and I can't find a solution to my problem.

I am using a trial professional account with 3 users. CEO and 2 sales representatives. 

The sales representatives have the same role in the hierarchy. I want them to be allowed to share contacts or accounts if they want to, but I don't want to use the default option to share all contacts and accounts. Only specific ones which they will work on together.