As a DM who handles multiple groups in one D&D campaign I like to check which players are on what posting group. It's quite time consuming that i have to hover my mouse over one player to see their additional info, the old version I can see every details i needed to see in the cast list without hovering. IS there a setting to change this back to the old way?

Not that I'm aware of, although as a GM if you need a One-Stop-Shopping option to see all the players' groups listed in one place, you can just peek in to the 'Edit Groups' option of the GM menu. It'll show you every group and every character all on one screen.

That was a change made for the Responsive update when it was decided to remove groups from player's view.  See discussion in RPOL Development | New Site Design starting around #431 for further details.

As SunRuanEr correctly advised, GMs can still see a player's groups either individually by hovering to get the expanded view OR en masse by navigating to GM Menu | Edit Groups.