Because I like Excel WAY too much, I did this just as info. This will show the max level you can group with and still get experience.
Level Max Level
25 38
26 39
27 41
28 42
29 44
30 45
31 47
32 48
33 50
34 51
35 53
36 54
37 56
38 57
39 59
40 60
41 62
42 63
43 65
44 66
45 68
46 69
47 71
48 72
49 74
50 75
51 77
52 78
53 80
Level Max Level
25 38
26 39
27 41
28 42
29 44
30 45
31 47
32 48
33 50
34 51
35 53
36 54
37 56
38 57
39 59
40 60
41 62
42 63
43 65
44 66
45 68
46 69
47 71
48 72
49 74
50 75
51 77
52 78
53 80