With regards to SQL Server 2008 Express, see here.
There is no explicit limitation on the no. of simultaneous users, but you would need to work within the limitations of 1 CPU, 1 GB memory utilisation, and max 4 GB-sized databases (10GB for SQL Server 2008 R2).
SQL BAK Explorer - read SQL Server backup file details without SQL Server.
Supports backup files created with SQL Server 2005 up to SQL Server 2017.