The 4.3 update of Threema for iOS adds full support for mentions in group chats. Simply tap the “@” key in a group conversation, and select the contact you wish to mention. This way, even the most lively discussions remain comprehensible, and group members never miss a message that’s relevant to them again (cf. “Notify when mentioned”).
On top of that, this update packs several other additions and improvements. For example, you can gently press on a link to open a preview of the respective website (requires a 3D Touch device).
For all the details, please refer to the changelog.