Working with Group Chats
One of the most common questions I get asked is about group chats/threads. Hopefully this wiki page helps others in their quest to do battle with Skype and its group chats. So, in no particular order...
Right-click a buddy in the buddy list and choose the "Initiate Chat" option. You can invite more people to the chat using the Conversation->Invite... menu.
The easiest way to do this is to, from the buddy list, Tools->Room List. Then pick your Skype account and then the Get List button. The /list command in a Skype chat/IM window will also get you there.
You should end up with a screen that looks something like
You can then click on the chat, then click either the "Join" button to join the chat or the "Add Chat" button to add it to your buddy list for quick access later.
When Microsoft bought Skype in 2011 they started to transition people over from old peer-to-peer (P2P) group chats (that have ID's like #person1;$person2;argleblarglefoobarwibbleblarg) to newer cloud-backed thread's (that have ID's like 19:[email protected]). Any chat started before 2011 would have been an old P2P chat, whereas generally chats started after this time (depending on when Skype clients were updated and stuff) would be a new Thread. This plugin only works with Threads, as that's what the web interface of Skype works with (the old plugin used to only work with P2P chats). You can create a new Thread with all the same people copied over from the P2P chat by running the /fork command in the P2P chat, in your regular Skype client (or mobile phone or whatever)
P2P chats rely on people sending messages to each other and needed everyone to be online to get messages. Threads store the messages on a Microsoft server and forward the messages around to work offline and are more-easily shared between your multiple clients/devices.
libpurple - the library that powers clients like Pidgin, Finch and sometimes Spectrum and Empathy - needs to use these ID's to know where to send chat messages to. You can hide these by saving the chat to your buddy list, using the Conversation->Add... button, and then Conversation->Alias to rename it to something nicer.
You'll need to use the /leave command to permanently leave the chat. Normally, libpurple would leave the chat for you when you close the conversation window, but that's not really how Skype does it, and if you're coming from Skype-land then you're probably expecting it to behave that way.
Other than /leave to permanently leave a chat, an admin can: /kick to kick a naughty user out, /add to add a user, /topic to set or unset the topic