LINQ: How to get the Max Id with a group by clause?

You want to pick top record from each group.

var ProcessAudList = ProcessAudService.Where(x => x.Active == "Y").GroupBy(x => x.ProjectSeq, (key,g)=>g.OrderByDescending(e=>e.ProjectValue).First());

Check demo code

When you use GroupBy extension, method will return you IGrouping instance and you should query IGrouping instance like below;

var ProcessAudList = collection.Where(x => x.Active == "Y").GroupBy(x => x.ProjectSeq).Select(x => x.OrderByDescending(a => a.ProcessSeq).FirstOrDefault()).ToList();

Hope this helps

You're most of the way there, but Max is the wrong term to use.

Each IGrouping is an IEnumerable (or IQueryable) sequence of its own, so you can use OrderBy and First clauses to get the answer you need:

var ProcessAudList = ProcessAudService    .FilterBy(x => x.Active == "Y")    .GroupBy(x => x.ProjectSeq)    .Select(grp => grp.OrderByDescending(x => x.ProcessSeq).First())    .ToList();

The Select clause will process each of the groups, order the groups descending by ProcessSeq and select the first one. For the data you provided this will select the rows with ProcessSeq equal to 12 and 14.