There is definitely some confusion here.  If you have a true single tenant application (Multitenant is No for all eSpaces) then there should be no issue as there are no tenants.  You can have as many eSpaces as you want and still be single tenant with a single user base.  So here's what you need to check.

1) All eSpaces are NOT multitenant
2) All eSpaces have Users as the User eSpace
3) In any eSpace under Roles, create the roles you need and make them public so you can use them everywhere

You can then use the Users application (your site/users) to manage groups and users.

Hope this helps,
Curt

Thanks Curt,

It makes completly sense for me.
But I think there is something wrong I'm doing, since when I create 2 eSpaces in the same application, 2 diferent lines (TENANT_ID) are created in the OSSYS_TENAT table.
Is that normal?

For example:

select ID, NAME, IS_MULTITENANT from dbo.OSSYS_ESPACE where ID in (1, 2)

* IDs are fake

Thanks again.

I have no idea why you are looking at the system tables like you are but if you just follow the steps I provided it should fix the problems you are having.