Have you tried this link first: Set Permissions for a Project

For the particular User that you want to be able to see the Project make sure the "View" box is checked in the Project permissions rules:

In order for the other User to not see the Project you can do a couple of things.  You have the group All Users by default and you can leave that box blank (it will default to Deny in the absence of other rules) or you can specifically make it as Deny...I wouldn't do that, it's best to leave it blank.  Optionally, you can add the other User and set a permission that specifically has Deny in the "View" permission.

agustin leira wrote:

 

...Always I must put an user in a group and then give the permissions to the group of the user?

No. 

First, a User can be in zero to many Groups, meaning they do not have to be in a Group.  A Group is simply used to hold a bunch of Users that is used only for permission rules within a Project.  It's easier to set the permissions once for a Group -- especially if it is in more than one Project! -- than for every time a User is added or deleted from the permissions of one or more Projects.

Second, you can set the permission for a User, and/or for the Group the User is in, and/or the default and always present All Users group.  Remember, the All Users will always be present and it's okay to just leave it all blank (basically ignore it) as other permissions will have a higher precedence and if none of the permission rules apply then the default permission will be Deny (if you left it all blank).  Yes, naturally if you make changes to the All Users permissions then whatever you have set them to would be the default permission should any other permissions not apply.

Read this as well:  Set Default Permissions at the Project Level

In fact, read everything you can about permissions from the admin. guide.  I admit that it's not super simple and sometimes I have to go back to the documentation to figure things out

Read this as it helps you figure things out, especially if there are several permissions that could apply to a User: How Permissions are Evaluated   In fact, the whole section this is in about permissions is worth reading.