Create Office 365 Group and assign users using Graph API

Check below for sample code to create Office 365 group and add user to this group using MS Graph API:

var newGroupName = "TestGroup";
var newGroupMailNickname = "TestGroup";
var userEmail = "userEmail";
var group = new Microsoft.Graph.Group
{                        
    DisplayName = newGroupName,
    GroupTypes = new List<String>()
        {
            "Unified"
        },
    MailEnabled = true,
    MailNickname = newGroupMailNickname,
    Description = "Group Description",
    SecurityEnabled = false
};

var createdGroup = _graphServiceClient.Groups
    .Request()
    .AddAsync(group).GetAwaiter().GetResult();

Microsoft.Graph.User userToAdd = _graphServiceClient.Users[userEmail].Request().GetAsync().GetAwaiter().GetResult();                    
_graphServiceClient.Groups[createdGroup.Id].Members.References.Request().AddAsync(userToAdd);