Currently, when creating a new user, you can not add him to a tenant at that step. So you can not log in to the new user because of that.
For a better experience, the action in the cli and ui should allow an optional option of a tenant for creating a user.
Example:
cfy users create <username> -p <pass> -t/--tenant-name <name> -l/--user-tenant-role <role>
Can be combined with:
cfy users create <username> -p <pass> -t/--tenant-name <name> -l/--user-tenant-role <role> -r sys_admin