There is a "move to last group" button, so you can select the top-most RCS, then click "move to group" and select the group you want them in, then click on the second RCS and then just start spamming the "move to last group" button and it will go down the list moving them all for you.
I tried that and while it works, some times it moves to the current group instead of staying in the same RCS group
One "trick" I use is to auto-group (if not already set up groups). Then I delete a group so the devices are put in "Ungrouped". Then I add the first device to a group, for the rest spam "move to last". Only if do "add to group" on the last entry in Ungrouped, then the selection will jump to the group - unless some buggy behavior.
Screw Auto Group, thats the biggest crap ever! At least when you du custom asignments (singlas, custom Signals), or have your Lights sorted according to Rooms... Or assigned the other halv of your 16 RCS to be activated with the other half of the thrusters to compensate for HG or MV, while beeing power saving… Auto sort screw's it all! As Long the Group you move your item OUT of, is the lowest one of the list, you can spam the 'add to last group'. I often use 'search' to Limit the items i'm seeing.
Auto Group only "works" if do not alter group names or not use signal logic. I try wait with grouping util mostly finished with the build. Then if enough devices, then I actually find it faster and easier to use the "trick" I posted. The way to "move" a group below another is to rename, since sorts A-Z (where Auto Group names is a bit special, just to have it mentioned).