We want things to be super easy, but current limitations mean that looking up a fellow Guardian on Steam is a bit of a challenge.
Instead of their gamer tag (which isn't unique), you'll need to use a SteamID64. What's that? It's an easy to remember 17 digit number (we joke, it's not easy at all).
To find your own SteamID64, just open Destiny 2 via Steam, then type
/id into chat. Easy!
This isn't quite as simple. Start by finding the other player's Steam profile page.
Next, copy the page's URL and paste it into this tool. It should output a bunch of info, including the SteamID64.
NOTE: To grab a profile URL from within the Steam app, you need to open Settings > Interface, and turn on "Display Steam URL address bar".
Good question!! Steam gamer tags are not unique, and they do not currently provide an easy way to look up accounts.
We will continue to look for a better way to handle this, and update the app if we find anything. If you have any ideas, feel free to log an issue.