Hi! So there are a couple of factors that can influence the bonus attributes(attribute enhancement) you can get from Group summons.

Now before anything, if you are in the same group, the summon should give you all the same "Group Summon Total Attribute Enhancement".

The value for "Group Summon Total Attribute Enhancement" is based on the total Bonus Attributes you get from each of the summons available: Katsuyu, Gamabunta, and Serpent.

An example can be found below:

The bonus attribute that you get from your Group Summons is based on the maximum level of your Group Summon as well as the % of Chakra it has, which can be found below:

Note: the "attribute" it refers to above is the Group Summon's battle attributes, and the "Bonus Attributes" is referring to the "Attribute Enhancement" that effects Group Members.

Remember, a group summon eats, so it consumes 7% of the total of his Chakra at 5:00 am (your server time) daily (to a maximum of 700)

This means your group has to sacrifice Chakra to the group summon to keep up its Chakra, as the Stage at which your Group summon is at will affect the "Attribute Enhancement/Bonus Attribute".