あなたのサークル’s direct translation is “your circle”. The サークル/“circle” here is somewhat a wasei-eigo (a japanese expression based on English), meaning a group of people of the same hobbie. In today’s context, the circle refers to a group to join the coming doujin event Comiket 100.

Doujin culture is an important part of the entire otaku market. Long story short doujin is the collection of amateur and/or derivative creations. (More about doujin on wikipedia.)

Comiket is short for Comic Market, one of the greatest doujin events in the world, which, usually, takes place twice a year. It’s basically a gigantic doujin party with events like cosplay, doujinshi (doujin creation) sales, and commercial campaigns. (More about Comiket on wikipedia.)

Today, the circles that applied for the coming Comiket 100 this summer receive the announcement of whether they are admitted and where they will be seated. In the announcement there is this phrase “あなたのサークルは…” followed by the details of the application. Circles then tweet about it on their accounts. Due to the great influence of Comiket, the tag goes trending.