Just a small listing to help you find out the max level a character can group with and still get xp.
Lowest Level Highest Level
10 15
11 17
12 18
13 20
14 21
15 23
16 24
17 26
18 27
19 29
20 30
21 32
22 33
23 35
24 36
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
Lowest Level Highest Level
10 15
11 17
12 18
13 20
14 21
15 23
16 24
17 26
18 27
19 29
20 30
21 32
22 33
23 35
24 36
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