SQL - alternate to SA to create accounts?

I'm attempting to set up a proof of concept for my company with SQL used as the backend for shared session storage. When I attempt to create new users with credentials other than SA in the Data Sources for the SQL DB, I get an error "Unable to save the user in the database!", "User does not have permission to perform this action". I've tried this with an account set to 'administrator' within Remote Desktop Manager and an account that was dbo over the SQL DB at the SQL server level with the same results. The only way I appear to be able to create new users is by using the SA account credentials in the Data Sources config. I would prefer not to use the SA account if that is at all possible. Is there a permission that I'm missing where I should add the account that I'm trying to use as the 'administrator'? It appears I'm able to create groups and only not able to create new users.

Thanks for the help and great product!