The activity we engaged in was good for she and I.
or
The activity we engaged in was good for us both.
or
The activity we engaged in was good for her and me.
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Sign up to join this communityPrepositions in standard English take the prepositional (objective) case.
The activity we engaged in was for her and me.
A good rule of thumb is to omit each pronoun in turn and see what fits
The activity we engaged in was good for her
and
The activity we engaged in was good for me
hence
The activity we engaged in was good for her and me.
though
The activity we engaged in was good for us.
can sometimes be used (where "us" is unambiguous) to avoid the problem.
or
The activity she and I engaged in was good for us.
(because "I engaged in an activity" and "she engaged in an activity")