All users are allowed to perform directed call pickup (if they know the * code that is).
The call pickup * code can be changed in the Registrar service settings (System -> Servers -> SIP Registrar). By default it is *78.
To pickup a call ringing on an individual phone dial *78 + Extension. So if extension 210 is ringing, you can pickup that call by dialing *78210.
Group call pickup is when you want to pickup a ringing phone out of a group of phones and you don't know the extension of the ringing phone.
For this feature, sipXecs uses hunt groups to define the group.
Create a Hunt Group (Features -> Hunt Groups), define an extension for the hunt group and put the phones in the hunt group.
Now if any phone in that hunt group is ringing you can pick it up with *78 + Hunt Group Extension. So, if the hunt group is 550 and extension 210 happened to be in that hunt group and it was ringing, you could pick it up with *78550.