If it's not the current event and you mean the Seven Signs event I don't think you can sign up this week as it's the second week (the event is a two week period). During Monday/Tuesday next week visit the Dawn Priest (or Dusk if you want to sign up for that) and you should be able to join up.
It sounds like you want to sign up for the Seven Signs. You can sign up on the odd-numbered servers this week. For even-numbered servers, you have to wait til next Monday.
The folks you talk to are the Dusk Priestess (to join Dusk) and the Dawn Priest (to join Dawn.) These folks are not marked on your map. One (Dawn usually) hangs out near the Church in whatever town you are in. The other is not in any set location. In Gludio, the Dawn guy is by the church, near the north exit to town, the Dusk priestess is near the grocery store. In Giran the Dawn guy is on the Church steps, to the south a bit, Dusk is on the west side of the statute in the central square. Neither appears in any starter village.
For other towns, I think /target works. (/target dawn priest)