can not see the tree

some of those I work with can not see my tree, just my profile. What can I do about it? Can anyone help me?

Define "work with". There are two options:
1) Are these people who are actually closely related to you, or invited to join your Family Group?
2) Are these Collaborators?

Family Group members should be able to see the part of your tree around you.
Collaborators can NOT see this part of the tree, or more specifically, they can only see your PUBLIC profiles, which are generally of deceased people (unless you manually set living people as public). As the people "around you" are, for the most part, alive (presumably), only actual relatives can see and/or edit them.

To see your private profiles you have to add them as friends for view-only access or extended family group member for edit access.

It is by the way normal not to be able to see the tree of a collaborator. You are collaborating on public profiles, not private profiles.

This applies to collaborate the person in question is my 20th Cousin once removed. Shall I then add the person to the family group or as a friend?

Liss,
do you want them to be able to just SEE the tree or also edit the profiles? A friend can see only.

Personally I don't see any reason why I should see the family tree of my distant collaborators or edit the close family profiles.

May be you should ask why?

If it is a question of not seeing your more distant profiles you should check if they are set as public profiles.