To get involved with someone - sich mit jemandem einlassen
To get involved with/in something - sich auf etwas einlassen
This sentence is using the auf + accusative form for getting involved with someone, so does this mean that both the mit and auf forms can be used for getting involved with someone?
Yes," sich auf jemanden einlassen" is also possible.
However, I would use it slightly differently.
“Sich mit jemandem einlassen” (bold corrected) always means entering a personal relationship, let it be a marriage or bank robber gang.
“Sich auf jemanden einlassen” can be less personal and fundamental, like believing a story that someone tells.
As so often the difference is gradual and not always clear cut.
Thank you very much for your help, it’s always nice to know these subtle differences.
I’ve now got a mental picture of Boris Becker saying the auf version of this sentence to his accountant (as he is led away to jail in handcuffs), and using the mit version with his ex-wife after concluding the divorce settlement.
As a side issue, I note that you used the phrase “mit jemanden” rather than “mit jemandem”, so how common/acceptable is it to use this form for the dative?