I will unsubscribe the moment this game happens. Ironic? Yes.
I have seen how cross server grouping has killed social interactions among the server community in other games. I will therefore help kill it all the faster by unsubscribing if it goes this route.
Legacy should do a bit more to inspire players to create alts. Perhaps make some legacy options permissible only with 2, 3, or 4 characters, not just legacy level.
This way there will always be people looking for groups.
Other than that; try to find groups at your server's peak time. You should have success.