Yammer (Twitter like messaging system)

I just came to know about “Yammer” from Techcrunch blog. Everything looks so clean and user friendly to me. I am currently using “Twitter”.

1. I couldn’t find any option from where I can invite people from different company/domain.
I think we should be given this option of inviting people in different company. Because people like me have 100s of contacts outside the company.

2. I believe no company allows you to access company mailbox after you leave them. So does it means that my yammer account will be of no use (as I won’t get any mail updates) if I changed my company. What about my old friend in previous company?

3. Most of us switches companies yearly. Is there anything which combines two or more profiles because these days nobody wants to maintain two account for same purpose.