Equivalent of CHARINDEX OR IN for ints
You haven't really indicated how this is called.  Do you really have a string full of comma separated ints or is that just an artifact of some other aspect of your system?

If the outer part of the environment around this requirement is changeable, then typically in SQL Server 2008 and up, I pass a table-valued parameter containing a list of IDs (perhaps from ADO.NET or whatever) and then just use it as a table inside the procedure - i.e. WHERE value IN (SELECT id FROM @table) or INNER JOIN @table AS t ON t.id = users.t