Out of the box Redmine doesn't look like it has a lot of options for managing clients, but you'd be wrong - with a little customizing you can at least fake it. There are two parts to this - set up a new user group and add some custom fields for users.

The first step is to create a new user group called Client:

• Log into the Administration -> Roles & Permissions page and click on New Role
• Set the name to "Client" and give them the follow permissions: Issues - add, add a note, change status, new file, new relation; News - add a comment; Documents - add, new file; Files - add. (adjust as you see fit)
• Save what you've set and assign a user the Client role for a project.
• Next time you view that project's Overview page it'll list them on the Members block as a Client

Simple but useful.

The second step is to round off the user information options by adding some custom fields.

• Go to the Administration -> Custom Fields page and click on Users link.
• Click on New Custom Field, set the name to "Company", leave the format as "text", set the min length to 0, the max length to e.g. 30, leave the regular expression field blank, leave the required option unchecked, and click Save.
• Repeat for any other fields you want, e.g. Work Phone, Cell Phone, Work Address, etc.

Now any time you add a user you can give them the above information.

Putting these two together, now when you go to the Project Overview page you can see who the client is and by clicking on their name see their contact information.