Sometimes it is hard to spot syntax errors in strings built this way. Try storing the SQL string in a variable and then looking at what got stored.
dim strSQL as String
strSQL = Select * From [users]where [user]= '" & Forms!usersignin!User & "'"
Set RST = db.OpenRecordset(strSQL)
Put a stop on the set statement and look at the value in strSQL and see if you can see the syntax error. Open the debug window and print it if you can't see the whole thing when you put your mouse pointer over the variable. There is no space between [users] and where. That could be the problem.