I didn't think Shift 1 used activation, at least not the DVD version I bought, but Shift 2 Unleashed does. For Shift 2, activation is handled by a different server, solidshield.com. You get up to 5 activations, and these can be on separate systems. The same key used for activation is also used one time (only) to associate it with an EA account. So in the case of a used copy of Shift 2, you can probably install and activate it, but you won't be able to use that key with a different account than the account it is already assigned to. You may be able to ask EA support for a new key, which could be used both for activation and to register with an EA account.
If you bought a Steam version of Shift 1, it appears there may be issue with online registration. You'd have to ask EA support for help with this.
There's hardly anyone playing online any more for either Shift 1 or 2, so unless you setup a time with a friend to play online, you probably won't find others to do multi-player events with.