There is no solution on this unless there is solo mode.
Even if there is no exp bonus for grouping, a minimum 2 players will still outclass solo player in term of levelling or farming.
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2 people will clear the low level GR faster than solo mainly because they can split. Faster clear, more run, more exp per hour.
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2 people will clear the Bounty faster than solo mainly because they can split.
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2 people can clear higher rift faster/better than solo because of team synergy, role assignment and better survivability.
So yes, 4 people will be even faster and effective than 2 people group. Either you get a friends to play together or just deal with it that solo will never outclass the group play.