The goal is when calling a contact or company, I can easily see in which projects they were involved in the past, and which collegue(s) were involved. These are all projects where at least four contacts from different companies should be visible.  

Closed Events might be the best place for it, but standard I can only add 1 account and 1 contact here. I didn't succeed in adding custom fields that refer to 'contacts' or 'accounts'. A custom field as a text area is possible, but I would rather refer directly to contacts/accounts. 

Any tips?

I am using CRM Free edition at the moment (Dutch translation, switching to English didn't seem to give me more options).