1. Connect the device to itunes and get UDID

2. Share the id with the developer to add in the dev account

Also, We can do it by  accepting the invitation shared by Developer.

How to get UDID of a device:

We can get it through itunes or itools

iTunes:

1. Connect your device to PC

2. Launch itunes

3. Check if the device got recognised

4. Click on Device details twice

iTools:

1. Connect your device to PC

2. Launch itools

3. Check UDID option under device details panel