well, I guess you could do this a couple of ways...
you could go to the 'all assets' page/section, filter if possible, and check the ones you want, and click the 'Add to Group' button on the left menu, and select your group...
or, you can do it the unsupported way (as in Lansweeper Support will say 'nope - you're not supposed to do that, you shouldn't have taken the advice of JacobH on the forum, and you need to restore from backup) by creating a report that gives you the assets you want to add, make sure it has assetID on it, export it to excel, do a select * from tblassetgroups, take note of the AssetGroupID, then insert a record into tblassetgrouplink (specifically assetids from the excel report, and assetgroupid from the previous lookup you took note of...
If you don't know how to do a SQL INSERT INTO statement, ignore my last suggestion... plus, you run a chance of messing up your database/integrity... and always make a database backup prior to doing it, or have a backout query handy. This method would take a few seconds... but in a few seconds you could jack up your data as well.
Hope that helps