Deleting rows in a table with Linq to SQL

removeFromGroup is still of type IQuerable.

You need to specify an actual GroupMember to delete.

You could use

GroupMember removeFromGroup = (from gm in dataContext.GroupMembers
                               where (gm.memberID == memberid) && (gm.groupID == groupid)
                               select gm).SingleOrDefault();


dataContext.GroupMembers.DeleteOnSubmit(removeFromGroup);
dataContext.SubmitChanges();

Alternatively, if your query returns a collection (from the looks of it, it won't since you are filtering by memberId) you could use

List<GroupMember> removeFromGroup = (from gm in dataContext.GroupMembers
                                     where (gm.memberID == memberid) && (gm.groupID == groupid)
                                     select gm).ToList();


dataContext.GroupMembers.DeleteAllOnSubmit(removeFromGroup);
dataContext.SubmitChanges();

You have to call .ToList()

var items = removeFromGroup.ToList();
foreach (var item in items)
  dataContext.GroupMembers.DeleteOnSubmit(item);

For batch deletes I use this, because LINQ to SQL first loads the entire data which is going to be deleted, then it does the deletes each by each.

https://terryaney.wordpress.com/2008/04/14/batch-updates-and-deletes-with-linq-to-sql/

https://github.com/longday/LINQ-to-SQL-Batch-Updates-Deletes

var listToRemove=(from a in DB.Table
             where a.Equals(id)
             select a).ToList();

DB.Table.DeleteAllOnSubmit(listToRemove);
DB.SubmitChanges();