I have an account with a cloud storage provider and I want to know if is possible to use it with two diferent users (each one with your own SMEStorage account).

I use a Personal Lifetime Cloud and the other user a free account.

I've tried creating a Group, inviting the other user and share with him a folder. I assigned (to the group and to the user) the permission to submit. The problem is that the user can't upload files to the group.

What am I doing bad?, Is it possible to do what I want?

The way groups works is that once created they become a 'view' on files you share. The files remain on the same place they were stored but are available in the view. For the recipient  they receive a group share link and are able to access these files under the group node in the File Manager.

If when you create the group you gave them permissions to submit they will be able to right click files on any storage cloud and choose to also share with the same group so that they can also add files to the group view.