Since the members are most active on Twitter, that's where the fandom centres too, although you can also find fans on other social media sites like Instagram, or in group chats using LINE or Discord. There are lots of creative irumii who post fanart, translations, and covers of MeseMoa. songs, especially to commemorate certain anniversaries or events. Some fans also publish the setlist and reports of their observations during performances or meet and greets.
If you're looking to find friends within the irumii community, check out the IWantToMeetMoreDDFans tag on Twitter! The Japanese equivalent is the イルミィさんと繋がりたい tag. Most Japanese irumii are very friendly and happy to speak to overseas fans, and some of them even travel to their events abroad. However, you should be aware that Japanese Twitter etiquette is a little different, as some people don't like it if you follow them without introducing yourself first.
kaigai irumii (海外イルミィ) - The name for MeseMoa.'s international fans.
odottemita (踊ってみた) - Literally meaning 'tried to dance', this is the category under which dance cover videos are posted to NND.
odorite (踊り手) - Somebody who posts odottemita.
utattemita (歌ってみた) - Literally meaning 'tried to sing', this is the category under which vocal covers are posted to NND.
utaite (歌い手) - Somebody who posts utattemita.
oshimen (推しメン)- Your bias or favourite member of the group. Sometimes shortened to oshi.
oshi (推し) - Often used with member colours to describe fans of a certain person, e.g. chaoshi (茶推し) are Nozakibento biases. An exception to this are Kimagure Prince fans who call themselves oukokumin (王国民) meaning 'subjects of the kingdom'.
otaku (オタク) - Not just a fan of anime as it's come to mean among English speakers, but a diehard fan of anything, be it sports, trainspotting, or idol groups. Sometimes also shortened to wota (ヲタ).
zawazawa (ざわざわ) - Onomatopoeic word for rustling sounds. The members post this to drum up excitement just before they're about to post a new video or make an announcement.
wwwwww - Any stream of w's in a row indicates laughter. It comes from the verb warau. You may also see (笑) or 草 being used.
88888 - Used to indicate applause. The pronunciation of hachi (8) sounds similar to the onomatopoeic word for clapping.
You can also find other guides to Japanese internet slang and fandom terminology such as this one online.