some suggestions

[Aaronock]

Majority of players stay at Prontera

Like what Panda said, a lot of guilds have "headquarters" or whatever you want to call them. Depending on the time of day you might not see many of these players in those towns but at other times several of them might be there.

Events/Activities suited for characters in levels 40-70

Novice events actually happen quite a bit, and any character can go to the Werewolf RP event. Also many will breeze through 40-70 and get to somewhere in their 80's before they start complaining about it being "slow." I am pleased they also cater to the higher levels. Also do try to party, a lot of lower level characters and new players don't want to do much alone, and I understand, you want to meet people

As far as guilds go, there are tons, and you can ask in the Guild Section for people to come to you and tell you how awesome their guild is for you!