Any Skype registered user can participate in a conference call. The host of the conference call, who is the person initiating the call, has to invite the different contacts to the call. Once they accept, they are in.
Can someone who is not invited join? Yes they can, as long as the call host accepts. They call the host, who will be prompted to accept or reject the call.
Also, people not using Skype, but using another phone service, like a mobile phone, a landline phone or a VoIP service, can join a meeting. Such user will of course not have the Skype interface and not use their Skype accounts, but they can dial the host’s SkypeIn number (which is paid). The host may also invite the non-Skype user using SkypeOut, in which case the former incurs the calling cost.