Reflexive (using »mich, dich, sich, ...«), with the preposition bei and with the object in dative case:
sich bei jemandem melden = to contact someone
Non-reflexive, without a preposition and with the object in accusative case:
jemanden melden = to report someone
Ich werde mich am Freitag bei dir melden.
I will contact you on Friday.
This means, that I will call you on Friday, or I walk to your house/office and try to get in contact with you.
Der Verdächtige darf das Land nicht verlassen und muss sich jede Woche einmal bei der Polizei melden.
The suspect is not allowed to leave the country and has to contact the police once a week.
Technically this also means, that the person just has to call the police once a week, but I think what really is meant, that this person has to appear in a police station to prove that he has not left the country.
Ich habe gesehen, wie du das Geld genommen hast. Ich werde dich bei der Polizei melden.
I saw how you took the money. I will report you to the police.
This means, that I will call the police and tell them who took the money.