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Oh yeah, people are still playing Sins online, though sadly the player counts are lower than what the game deserves. Still, you should be able to find games during North American prime time and on weekends. It will be easier if you have Entrenchment though you might be more likely to find newer (to online) players in Regular Sins. Your best bet for finding newer players is to look for games with "Normal" speed settings and also on the weekends.
Just log onto ICO and dive in.
Do be aware that regardless of how good you are against the AI, that the online game against human opponents is much more challenging and that as a new (to online) player you will get beaten a lot while you learn how to play against other people. (Even the pros get thumped down by other pros. In a two team game, half the players are destined to lose.)
I think that as a new player, at least in the team game, it helps to try to frame your criterion for having fun a little differently. Think of your goal in the game as that of not having to defeat your immediate opponent but rather that you want to remain a factor in the game for as long as possible and to be as big of a pain-in-the-ass as possible. You just want to survive and to occupy whoever is beating you up as long as you can so that your allies can win their battles without your assailant joining his buddy for a 2v1 against your ally.
So, for example, you are not obligated to throw your fleet away when it's obvious that your opponent's fleet will wipe out your fleet. It would be better to just avoid combat and either migrate to the middle of the map (preventing easy colonizations and forcing the bad guys to have to bomb those planets out to increase their holdings) or to hit the guy who is attacking you in the back with your force so that he has to retreat his ships. Then just continue to use a hit-and-run approach. That would help your team much more than just getting wiped out completely. (After all, if your team wins, you win even if you lost your battles.)
One of the mistakes new players often make is to allow themselves to get decisively knocked out of the game completely (no planets, no colonizer frigates or colonizing capital ships) rendering themselves a non-factor. So the first strategy you're going to want to learn is simply, how to survive and buy time for the heavy hitters on your team. What can you do to climb back into the game and to remain a factor. (A guy with only 2 planets in the middle of the map and 2 capital ships and 20 long range frigates can attack someone an ally is fighting, causing a 2-on-1, remaining a factor in the game.) Do note that if you have to migrate, you will need to designate a new home planet at some point (or you'll never have a credit or resource income).
Alternatively, you could try to hole up at your home terran with a starbase and supporting defenses (repair bays, hanger bays, flak frigates, your own carriers with fighters, etc.) and buy time. (This is probably your only option for buying time for your allies in a 2v2 or 3v3 if things start going badly for you.)
If you try to think about the game that way, it might be more fun. Note that the migration strategy really only applies to larger games--4v4 and 5v5. Smaller games such as 2v2 and 3v3 are much less forgiving since there probably won't be too many planets to migrate to and if you go down, it becomes 2v1 or 3v2 against your allies.
So try to play 5v5's if you can. Just introduce yourself to the game's host and tell him that you are new (and not a smurf) and would really like to play. You'll probably end up as last pick in the player draft (aka "fat boy") so you'll try to survive and buy time for the heavy hitters. (In the 5v5's people will go to Team 10, two people will become captains, and then they'll draft-pick the players in an A-BB-AA-BB-A order so that the first captain has the first and last picks. The idea is to get balanced teams and better games.)