Each is used if we mean an individual member of a group.
Every can also be used.

We have send the invitation to each / every business partner.

Each of is used if we mean every member of a group and we include a possessive pronoun our, their, etc.
Every cannot be used in this case.

We have sent the invitation to each of our business partners.

Every is used with a plural noun when it is followed by number.
Each cannot be used in this case.

I visit my mother every two months / every other day /every second week.