For /join or /invite you need to use the targeted player's id, instead of going to your steam profile and copying the id from there you can type /id in chat and it will show your id. It can save you a few seconds..
Typing /h in the chat will show you all the available commands.
External link →Yeah, it should be your new steam ID, so you dont need to see your numbers for it EDIT: I guess it doesnt change anything for in game in destiny, so that makes no sense
You can't change your numeric SteamId. The thing in the Steam profile you can change (other than your display name) is the profile URL.
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