There's no hard limit to how many users you can have on the system. The number of connections is not related to SQL Server edition, but to the operating system. For example, Windows XP allows only 10 concurrent connections, Windows 7 20. For Windows Server OSs, the number of connections is unlimited (but you can limit is using Terminal Services).
However SQL Express has other limits such as the maximum size of the database (4GB), CPU sockets (1) and amount of memory (1GB).
It really depends on how much data you have (remember the database size of 4gb is for metadata and transaction data, not actual solidworks file size). If you intend on having 7-8 people accessing it all at once and constantly working on it, I'd say it may be a little slow, but obviously the jump in cost to PDM Pro which would alleviate that is considerable.