Report to get the number of users assigned to issues in a project

In our company we use the issues section of a project as a repository of iniatiatives. Each iniatiative is an issue, which is assigned to one or more users. Also one user is assigned to several issues.

I want to know how many users in total are participating or in other words are assigned.

Does anyone knows how to do it or could provide a link to report in which I can get this info. I truly appreciate it.