string strgroupids = "6";

groupIds.ForEach((g) =>
{
    strgroupids = strgroupids  + g.ToString() + ",";
    strgroupids.TrimEnd(',');
});

strgroupids.TrimEnd(new char[] { ',' });

strgroupids = strgroupids.Remove(strgroupids.Length - 1);

strgroupids = string.Join( ",", groupIds );

public static string RemoveLast(this string text, string character)
{
    if(text.Length < 1) return text;
    return text.Remove(text.ToString().LastIndexOf(character), character.Length);
}

yourString.RemoveLast(",");

while (strgroupids.EndsWith(","))
    strgroupids = strgroupids.Substring(0, strgroupids.Length - 1);

var strgroupids = String.Join(",",  groupIds);

string strgroupids = string.Empty;

groupIds.ForEach(g =>
{
    strgroupids = strgroupids + g.ToString() + ",";
});

strgroupids = strgroupids.Substring(0, strgroupids.Length - 1);

string strgroupids = groupIds.Aggregate(string.Empty, (p, q) => p + q + ',');
strgroupids = strgroupids.Substring(0, str1.Length - 1);

string strgroupids = groupIds.Aggregate(string.Empty, (p, q) => (p != string.Empty ? p + "," + q : q.ToString()));

strgroupids = strgroupids.Remove(strgroupids.Trim().Length - 1);

var strgroupids = string.Empty;

groupIds.ForEach(g =>
{
    if(strgroupids != string.Empty){
        strgroupids += ",";
    }

    strgroupids += g;
});