Linking your Apple Developer Account to ours
Log into the appstore Connect (https://appstoreconnect.apple.com/login) and go to Users and Access section.
Click on the Plus button next to Users to add a user.
Email: firstname.lastname@example.org, First name: Disciple, Last name: App Store. Select option "Admin" for Role. Make sure all the role fields are ticked, select your app from the drop down underneath, make sure to click "Access to Certificates, Identifiers & Profiles" then click Invite.
Linking your Google Play Developer account to ours
Start by logging into your Google Play Console. Then visit the left hand menu and toggle Developer Account, where Users & permissions will appear underneath. Click there and select Invite New User as shown below as a blue button.
Fill in the form using email@example.com as the email, Access expiry date set to Never and the Role set as Admin. Make sure you type the email address in and not copy and paste it.
Select your app from the drop down menu next to Permissions and ensure all the boxes under Access Level are ticked before pressing Send Invitation.