I don't know how to describe this question... .... normally you can do (highest lvl / 2) + 1 or something to find out if you can group with someone and get XP. How does this work below lvl 20? I know a lvl 1 can group with a lvl 6 and get XP, but not with a lvl 7. How do you do it for a lvl 4 for example? What is the formula? Thanks.
5 levels difference is always ok regardless of the formula result at low levels. Here's a super handy chart.
I seem to recall it was something like, your level, plus half your level plus one level is what you can group with. So a level 5 could group with a level 8. (5+2.5+1 =8.5 (round down) This may have changed, but it seems about right.
Yes, except it's over-ridden by the fact you can group with anyone within 5 levels of you, and everyone gets XP. So, in your example, the level 5 can group with a level 10.