Trust EXP can only be earned between the Character assigned as Leader and the other four Characters within the group.
If you'd like to increase the Trust Rank between certain Characters, assign them accordingly to their respective slots within the same group.
Please note that the other four Characters within your group cannot earn Trust EXP between themselves.
In addition, you can appoint a Character as a Sub-Leader by allocating them in the second slot within the group. Sub-Leaders will accumulate more Trust EXP than the other Characters in the group.