What level players can be grouped?

The general rule for the levels of members of a group is to take the level of the lowest level player and add half of their level, rounded up, to get the maximum level player they can be grouped with and still get experience. So a level 30 player can group with a level 30+15 or 45 player and still get experience. A level 31 player could group with a level 31+15.5 or 46.5, rounded up to level 47.

There is also a maximum level difference of 30, so a level 64 player can group with a level 64+30 or level 94, and not 64+32 (96).

For missions there is a maximum level difference of 15, and you will not be able to get a mission which has a larger level spread.