Highlight the white text inside the block for hints ranging from vague to specific...
This is a standard crossword puzzle that uses clues which are straight forward.
Once you solve for the first "One of the group" clues, it should be fairly obvious what the theme of the puzzle is.
1 Across is "AGO"
1 Down is "AGILE"
7 Across is 'One of the Group', "PERCIVAL". That should be enough information to figure out the rest of the group.
Percival, Lancelot, Merlin, and others are all character's in the story about King Arthur (which also is the answer to 43 Across "ARTHUR")
The five-word phrase appropriate to the "group" is found in the next block of text. If you do not wish to be spoiled, please do not continue.
The group is none other than King Arthur's Knights of the Round Table. The phrase "KNIGHTSOFTHEROUNDTABLE" is found within the puzzle itself, starting with the last letter of 19 Across "ARK" and traversing the entire grid in a circle. For example, the word "OF" is found going down in the middle of 30 Down "ASOF" and the start of the word "ROUND" is found going backwards in 65 Across (ORE). Makes sense? The keyword is "Round".