How to do Entity Framework/Linq join in query with where clause expressions

Question

I have a Entity Framework query where the where clauses' change based on several selections.
Here is my code:

Expression<Func<CtCsGroup, bool>> where_expression1 = x => 1 == 1;
Expression<Func<CtCsGroup, bool>> where_expression2 = x => 1 == 1;
Expression<Func<CtCsGroup, bool>> where_expression3 = x => 1 == 1;


if (selections.SearchCSGroup.NoNull() != "")
{
    where_expression1 = x => x.CsGroup.ToUpper() == selections.SearchCSGroup.ToUpper();
}

if (selections.SearchUserId.NoNull() != "")
{
    where_expression3 = x => x.UsrIdn.ToUpper() == selections.SearchUserId.ToUpper();
}

if (!selections.SearchIncludeRetires)
{
    where_expression2 = x => x.CsRetire != "Y";
}

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                 .Where(where_expression1)
                 .Where(where_expression2)
                 .Where(where_expression3)
                 select new
                 {
                     UsrIdn = g.UsrIdn,
                     CsGroup = g.CsGroup,
                     CsRetire = g.CsRetire,
                     CsGroupName = "test" // n.CsGroupName
                 }).ToList();

I want to add a join to another table.

How do I add this to the above query? I have tried several different formats but whenever I add it, I get an error "(where_expression1)" ,"(where_expression3)", etc.

Thanks.

Here are some more details...

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                         join n in _infobaseContext.TblCsGroupList
                                    on g.CsGroup equals n.CsGroup
                         select new
                         {
                             UsrIdn = g.UsrIdn,
                             CsGroup = g.CsGroup,
                             CsRetire = g.CsRetire,
                             CsGroupName =  n.CsGroupName
                         }).ToList();

• and this one works without the join -

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                         .Where(where_expression1)
                         select new
                         {
                             UsrIdn = g.UsrIdn,
                             CsGroup = g.CsGroup,
                             CsRetire = g.CsRetire,
                             CsGroupName =  "test"  //n.CsGroupName
                         }).ToList();

• but when I add both the join and the expression -

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                         join n in _infobaseContext.TblCsGroupList
                                    on g.CsGroup equals n.CsGroup
                         .Where(where_expression1)
                         select new
                         {
                             UsrIdn = g.UsrIdn,
                             CsGroup = g.CsGroup,
                             CsRetire = g.CsRetire,
                             CsGroupName =  n.CsGroupName
                         }).ToList();

• Visual Studio is giving this error: Argument 2: cannont convert from 'System.Linq.Expressions<System.Func<IBDataServices.Models.CtCsGroup, bool>>' to 'System.Func<char,bool>'