Assign to an email group!

Start in Exchange Admin Center

Navigate to Groups under Recipients.

Select the appropriate section, and search the group you want.

A panel will appear on the right.

Click "View all and manage", then "+ Add members"

From here you can add member by searching for them.

If you get an "On Premesis" error, you will need to go a step further.

Add the user in Active Directory or Exchange classic.

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You can get to Exchange classic from the bottom of the exchange nav area

or... Click Here

From there, navigate to Recipients > Shared and search the desired email group by clicking the magnifying glass.

Click the link and a pop-up menu will appear.

From here navigate to mailbox delegation and add user to BOTH areas.

Now just click save and your done!