Best Way To Do A Dynamic Bulk Insert To A Table

My current project is creating a social network for the university I work for.  One of the features allows members of a group to send a message to all other group members.  Currently, I run a foreach loop over each of the group members, and run a separate INSERT statement to insert a message into my messages table.  Once the group has several hundreds members, everybody starts getting timeout errors.  What is the best way to do this?
 
Here are two suggestions I've received: construct one sql statement that would contain multiple INSERT statements.  It would be a large statement like:
 
INSERT into [messages] (from_user, to_user, subject, body) VALUES (@from_user, @to_user, @subject, @body);  INSERT into [messages] (from_user, to_user, subject, body) VALUES (@from_user2, @to_user2, @subject2, @body2);  INSERT into [messages] (from_user, to_user, subject, body) VALUES (@from_user3, @to_user3, @subject3, @body3);
 
etc...
 
Or, do the foreach loop in a stored procedure.  I know the pros and cons of sprocs versus dynamic sql is a sticky subject, and, personally, I'd prefer to keep my logic in the C# code-behind file.  What is the best way to do this is an efficient manner?  I'd be happy to share some code, if that would help.  Thanks for your input!