Hello is it possible to set for some users the tab People of the deig

So you want people to have access to design, but not be able to do anything? The only way to do that is to create a special group which would have view permissions to everything and also be in the designers group.

In the first step yes, I want that some people see who is in charge of tasks.
In future that they can work operate in some of this groups.
Is it possible?

Perhaps you can accomplish this use case by building administrative forms in Tempo to show who is in charge of tasks. There are quite a few identity management function that allow you to get members of a group, which you could display in SAIL Tempo forms, and not have a need to expose your end users to designer interface.

Which is the function that given a group name in input list member users?

First you can get the group by using the function, getgroupbyname. Then you can apply the the getdistinctusers function to get all the user members of that group.