Bonjour Kaouther,
I don't understand exactly what you want to acheive but will try to answer each of your question.
1:haw add this type to the portlet ,so when i create a new field i choose selection list as type.
If you are talking about an select input, then you just have to create a JSP page and add a select input. When using Eclipse IDE and create a portlet using the wizzard, it generates usually a view.jsp page. Just add your select input. I would recommend you use "aui" taglibs.
<aui:form>
<aui:select name="mylist">
<aui:option name="item 1" value="1" />
<aui:option name="item 2" value="2" />
</aui:select>
</aui:form>
2: how i will put list of site members in this list
List of members of a site I assume. Then, populate the list above (aui:option) with the result of list of User. You can get list of users using the UserLocalServiceUtil class. There are multiple ways of getting list of users, you "just" need to know which list you want.
PS: si vous préférez échanger en français, ouvrez un fil dans le forum français
