It's now possible to generate a link to allow someone to add you as a friend without you needing to accept a request. These links look like
http://s.team/p/user-id/token which redirects to https://steamcommunity.com/user/user-id/token
You can also access a profile using https://steamcommunity.com/user/user-id which will just redirect you to /id/ or /profiles/, depending on whether the profile has a custom URL set. These user IDs look like aaa-aaaa.
I don't know why it might be useful to convert a SteamID to friend-link user-id, but converting the user-id to a SteamID might be useful.
The friend-link user-ids are just the account's accountid (the lower 32 bits in the 64-bit SteamID, or the x part in [U:1:xxxx]), encoded in hexadecimal, with some character replacements, and with a dash added. Here are the replacements:
Hex = Letter
------------
0 = b
1 = c
2 = d
3 = f
4 = g
5 = h
6 = j
7 = k
8 = m
9 = n
a = p
b = q
c = r
d = t
e = v
f = w
To convert a SteamID to a user-id, encode the accountid in hex and perform string replacements for the above characters. The hyphen isn't necessary for a final URL to work.
To convert a user-id to a SteamID, remove the dash and perform string replacements for the above characters. That gives you an accountid which you can turn into a SteamID using a platform-appropriate SteamID library.
