You can join now. Here's a list of open polities:
{HRE}Province of Brandenburg
{HRE}Province of Brunswick-Launeburg
{HRE}Province of Cologne
{HRE}Province of Lorraine
{HRE}Province of Savoy
{HRE}Province of Bavaria
Kingdom of Norway
Kingdom of Novgorod
Cuman Khanate
Principality of Moldavia
Khwarazmian Empire
Kingdom of Lombardy
The holy roman states are considered vassals of Austria, but can become Emperor themselves in an election. Brandenburg, Cologne, and Brunswick-Lauenburg are probably in the best positions. The others are quite instable, save Austria itself which has a player.
The Kingdom of Norway and the Kingdom of Sweden just hard their monarchs married to eachother so you have that to deal with if that suits you.
Kingdom of Novgorod is in a pretty nice position far enough from the other powers to avoid conflict at the start but close enough to interact, with a lot of room for expansion. Probably overall the "best" start, but probably least interesting.
I dont have much to say about the Khanate, and moldavia is an actual vassal to Poland.
The countries in the east are not reccomended as they are entirely isolated; one of the players that was there left and the other is MIA, and its next to the largest concentration of creatures in the game right now. All in all, not a great place to start your first game. Or in general.
Kingdom of Lombardy isnt in a great position in terms of power and favor, being at the forefront of an invasion of beasts and the overall poorest country in italy. However, this could prove to be the most interesting choice of them all.
Or, you can create your own country in just about any white space so long as its not too big or too small. Also, make sure its feasible to exist in the 12th-13th century.
