SharePoint Online: Add TermStore Managers and Contributors using CSOM

TermGroupManagersContributors.cs

ClientContext clientContext = GetClientContext();

var taxonomySession = TaxonomySession.GetTaxonomySession(clientContext);

var termStore = taxonomySession.GetDefaultSiteCollectionTermStore();

var myTermGroup = termStore.Groups.GetByName("My Custom Terms Group");

//Add Group Managers
myTermGroup.AddGroupManager("i:0#.f|membership|[email protected]");

//Add Group Contributors
myTermGroup.AddContributor("i:0#.f|membership|[email protected]");
myTermGroup.AddContributor("i:0#.f|membership|[email protected]");

clientContext.Load(myTermGroup, group => group.GroupManagerPrincipalNames, group => group.ContributorPrincipalNames);
clientContext.ExecuteQuery();

Console.WriteLine("Group Managers: ");
foreach (var manager in myTermGroup.GroupManagerPrincipalNames)
{
    Console.WriteLine(manager);
}

Console.WriteLine("Group Contributors: ");
foreach (var contributors in myTermGroup.ContributorPrincipalNames)
{
    Console.WriteLine(contributors);
}