How do I join a 100+ soul die2nite village?

There is no way to choose the city you want to join. Well, there is, and it's Hero-only.

Here's what you can do:

• You either join one of the cities randomly and the city happens to be an advanced one, or
• You are in a coalition with an hero, the hero chooses to join a suitably empty advanced city and as a result "drags" the coalition in with him.

You don't need to have 100 SP for the second scenario.

This has changed.

As soon as a player amasses more than 100 soul points, all future lives will begin in towns inhabited by players who also have over 100 points (allowing the best players to play together).

the official Die2Nite help page

If you have 100 soul points you will automatically be assigned to a 100+ soul point town. Being a hero would just let you choose which specific one you wanted to join or choose to join a normal town. Without being a hero, you will always be assigned to a 100+ soul point town now.

Note that right now, there isn't a whole lot of difference between normal towns and 100 SP towns. The only way to recognize a 100 soul point town is that there is an attack command when you look at the citizens page for others in your town. Other differences referred to in the help, such as a larger map, have not yet been implemented in the English version. And if you were expecting that the caliber of play will increase significantly, you might be disappointed in that respect as well...