Here's a handy tool for close groups of friends and for families: Foonz, a free conference call service.
On the Foonz Web site, you set up your calling groups. For each name, enter a cell phone number or an IM address (or both). Place your contacts in groups (e.g., "Beer buddies," and so on) and when you want to talk to everybody in a group at once, you call the Foonz service and select one of your groups from voice prompts.
Foonz then sends SMS or IM messages containing dial-in numbers to your contacts. People calling from known cell phones get popped right into your conference. People from phones new to Foonz get an access code that they need to enter.
Anybody who has used a big-business conference bridge will get the concept. This is like that, but with additional features. For example, when the call is over, you can leave a message for the people you invited but who didn't make it. When they dial in, they'll get your message (like, "We've decided to meet at Moe's at 9 o'clock tonight"). And instead of awful elevator music on hold while a call is being set up, callers are put in the "arcade," a waiting room that plays your choice of music genre.
Considering that Foonz is free, it's hard to find fault with the service. My only real gripe is that you can't initiate a conference from the Web. You have to dial in to the service to do that. It'd be better if you had the option to invite yourself to a conference via the site. And not that it matters to users, but I found it odd that the conference call I just had with the Foonz CEO and a PR person was not run over Foonz.
Coming later to Foonz: integration with Web-based social networks, in the form of a presence indicator you can put on MySpace or Facebook pages. See also Meebo's group text chat and Stickam's Webcam rooms.