Promotional Membership

How do I set Promotional Membrship as a Deafult membership level upon members join in?

This is not standard functional. So you need change functional to realize this.

any idea how?

I just jive common way to move. I`m not prepare custom modifications at forum. You can edit join.php for example, or create own files. You must care about payments during joining if you want to realize this. Or you want offer just free membership levels during joining?

I'd also like to set promotional or regular membership for default one cause I'm offering the services free of charge. Any idea how to do it?