I think they got updated in the latest update. At least for me they did. So if yours haven't, then soon? You need everyone in the group to update.

This is copied from the latest Android beta changelist. For what it's worth one of my groups has been converted so it seems to be happening.

> Your Legacy Signal Groups will be automatically upgraded to New Groups so they can get in on the new features we recently released, like admins, @mentions, group links, and more. Upgrades don’t happen all at once or instantly, and some groups may not upgrade for a while.

Hi, Signal employee here. The latest builds of all of our clients support upgrading. You just need to make sure everyone in the group is on the latest clients, including their linked desktops. In the future, we'll add the ability to force a migration for scenarios where some group members are inactive.