//load the list of users by IDs and add them to the list of recipients 
var recipientList = new List<IUser>();
recipientList.Add(PublicAPI.Portal.Security.User.Load(3));
recipientList.Add(PublicAPI.Portal.Security.User.Load(4));
//load the message author 
var messageAuthor = PublicAPI.Portal.Security.User.Load(5);
//create attachments (a link to creating attachments is in the end of the article) 
var attachments = context.Attachments;
//create an empty list of information channels 
var channelList = new List<IInformationChannel>();
//load infochannels by IDs and add them to the list
channelList.Add(PublicAPI.Portal.Objects.Messages.InformationChannel.Load(3));
channelList.Add(PublicAPI.Portal.Objects.Messages.InformationChannel.Load(4));
//message subject 
var messageSubject = "Message subject";
//message text 
var messageText = "Message text";
//document attachments 
var documentAttachments = context.DocumentAttachments;

//send the message
PublicAPI.Portal.Messages.SendMessage(messageSubject, messageText, recipientList, channelList, messageAuthor, attachments, documentAttachments);