Jesteście dla mnie mili.

Jesteś refers to one person, so it can be followed by "miły/miła" (m/f) depending of the gender of the person you refer to. "miłe" (n) is possible, but referring to a person you are talking to with neuter form is rare

Jesteście refers to more than one person, it can be followed by "mili/miłe", depending on who is in the group. (if there is one man in the group - mili, all women miłe)