Login to ads.google.com using your Gmail Id & Password
Login IDType Login ID
Password**********
On the right side top corner click on the "Tools" button
Under "Set up" column click on "Account access" to proceed.
Under account access setting, click on the (+) button to add new user/admin.
Once you clicked on + button you will redirected to the invitation tab. Select "Admin" as shown below & add gmail id of the account you wish to provide access to & click on "Send Invitation" tab.
Once you sent the invitation. The added user will receive a notification on their gmail account.
Accept the invitation from Gmail ID to get admin access of adwords account.
After accepting the invitation you will then be prompted to confirm the mail id. Enter the mail ID of adwords account who's access is given to you & click "Continue"
Google Adwords Account Access Steps