We do not use OMEMO in the group chat anymore, however you can message the specific participants using OMEMO directly. For that, you will need an OMEMO compatible client (Psi, Dino, Gajim, Pidgin, Conversations, ChatSecure...) and server (https://compliance.conversations.im/test/xep0384/ - not always reliable; used to list Disroot as OMEMO-supporting when it wasn't). Please do not use creep.im, it requires filling a craptcha for anyone who wants to add you to their contact list. For Pidgin, OMEMO works through the lurch plugin, so install it and enable through Tools > Plugins. For Gajim, you need OmemoGajimPlugin. If you have a problem, message me and I will try to help.