Charisma gives 5% extra XP & CAPS for each perk point when completing events in a group. (Total 75% increase for 15pp, 'Theory' 125% increase for 25pp)
Intelligence gains 1.25%/or 4%/maybe 5% extra overall XP for each level. (Testing unfinished)

herd mentality +2 s.p.e.c.i.a.l in teams, -2 s.p.e.c.i.a.l solo
egghead +6 intel, - 3 strength -3 endurance

3-Perks used with mutations are:
Charisma - "Strange In Numbers" buffs mutations by 25% when in a team. (Requires total 1 pp)
Luck - "Class Freak" reduces negative mutations effects by 75%. (Requires 1pp for each 25%, Total 3pp)
Luck - "Stretched Genes" Stops random mutations gain, Stops Radaway from curing mutations. (Requires 1pp for half the effect, Total 2pp)

4-Perks used for XP gain are:
Charisma - "Inspirational" gives 5% bonus XP for each perk point up to 15%. (Total 3pp)
Charisma - "Magnetic Personality" at 1pp gain 1 Charisma for each teammate, excluding yourself. at 2pp gain 2 Charisma for each teammate, excluding yourself. (Total 6 Charisma for full team)

"The Path to Enlightenment" Event, +5% XP for 60 minutes.
"Well Rested" effect by sleeping on any bed, +5% XP for 120 minutes.

Any legendary armor with "Unyielding", It gives +3 bonus to all S.P.E.C.I.A.Ls for each piece, except Endurance, when low on health. (Included in Bloodied Builds), (Total +15 S.P.E.C.I.A.Ls for a full 5 piece set)