Campaignmonitor seems to be "permission based", so you cannot send out Invites since invitees gave no permission. A current user deciding to invite another doesn't cut it as "permission".
If permission based is OK for your needs, AWeber is top of the line (www.aweber.com)
I heard it at Startup School '06 - I think at the mixer the night before. I'm curious if any have the flexibility in financing the work that some of the law firms will extend. Thought one of the big four might be in that niche.