You have to give each user his own pickup group.

eG.

User 200 has the group "pickup_200"
User 201 has the group "pickup_201"
User 202 has the group "pickup_202"

Each user is active in its own group. And each user has his own group selected as an group-indicator (in his own PBX object).

And now you can distribute the groups to any other users. But without active option.

So you can decide freely who is allowed to pick up the call of another.