how to transfer group type to text meanwhile get all information of words

Use the group() function: https://docs.appian.com/suite/help/21.4/fnc_people_group.html

So you'd want group(1-groupname, "groupName") but there are other properties that can be returned (check the linked documentation)

i mean i want switch all the group type to text, toString just get partial

If I understand your correctly this may be what you are after:

concat(
  group(togroup(1),"id"),
  " - ",
  group(togroup(1),"groupName")
)

Not 100% sure what you're trying to achieve though. Could be totally misunderstanding.

understood, but is there a thing can transfer excel to CDT?

Yes You can , there is a smart Service "Import Excel to Database" use it to store Excel Data into DB. After that Query the DB & store result in CDT.
If this smart service isnt available in your env try installing Excel Plugins

Thanks, do you know which function can get group users' total number?

a!groupmembers this  function returns all the users from a particular group. you can use count function to get total count. Only drawback is this function can return max 10k users