There are two parts of access an agency will need.
1. Page access
2. Advertising account access
Here’s how to give both…
PART 1: Give Page Access
(You’ll need admin access)
1.Go to your page & click Settings at the top
2. Click Page Roles in the left column
3. Select the person you want to add:
If the person is your Facebook friend, begin typing their name and select them from the list that appears.
If the person isn’t your Facebook friend, type the email address associated with their Facebook account.
4. Click Editor 
to select a role from the dropdown menu
5. Click Save and enter your password to confirm
Depending on their settings, the person may receive a notification or an email when you give them a role.
Now they have page access. Nice. But usually we’re not done here.
Now we need to give advertising account access.
PART 2 : Give Advertising Account Access
You can give people access to your advertising account from your ad account settings in your Ads Manager:
Go to your Account Settings in Ads Manager. You may be asked to re-enter your password, so don’t be alarmed by this if it happens.
THIS LINK WILL TAKE YOU THERE:
https://www.facebook.com/ads/manage/settings.php
Scroll to the Ad Account Roles section, click Add a Person and then enter the name or email address of the person you want to give access to. Learn more about the different kinds of advertising permissions.
THIS LINK WILL SHOW YOU THE DIFFERENT PERMISSIONS:
https://www.facebook.com/business/help/155909647811305
We like to get ad account admin access, just in case we need to edit bits. But ad account advertiser works too.
Use the dropdown menu to choose their role and then click Submit. The person you add will immediately have access to your ad account according to the role you chose for them.
Note: Adding people to your advertising account doesn’t give them permission to log in as you or see things on your profile or Timeline that you haven’t shared with them.
All done, now we’re ready to rock!