Expression evaluation error at function 'group' [line 13]: The passed parameter(s) are of the wrong type. Received the type List of Group.

Also in case it helps, I have a similar task report which passes the value(s) in the "assignee" column into this rule, essentially "APN_displayGroupOrUser()", with a single rule input of type "user or group".

if(
  runtimetypeof(ri!groupOrUser) = 'type!{http://www.appian.com/ae/types/2009}Group',
  if(
    isnull(ri!groupOrUser),
    null(),
    group(togroup(ri!groupOrUser), "groupName")
  ),
  rule!APN_displayUser(user: ri!groupOrUser)
)

Also in case it helps, I have a similar task report which passes the value(s) in the "assignee" column into this rule, essentially "APN_displayGroupOrUser()", with a single rule input of type "user or group".

if(
  runtimetypeof(ri!groupOrUser) = 'type!{http://www.appian.com/ae/types/2009}Group',
  if(
    isnull(ri!groupOrUser),
    null(),
    group(togroup(ri!groupOrUser), "groupName")
  ),
  rule!APN_displayUser(user: ri!groupOrUser)
)

When there are list of groups like 2 groups  it doesn’t work . In expression rule it gives the group name for first group and at interface it through an error for invalid group exception

Where and how are you trying to handle the assignee elements?  By default the report returns an array of assignment values (person or group), so in my interface I iterate over all such entries using a!forEach.

I have used a! for each function  only but its giving me group id not name

Can you post the full context for what you're trying to do in your grid column, like I have above?

Are you now calling a helper rule similar to the one I showed you above?  Where is your a!forEach() statement, and how is it structured?

yes i am using the similar rule.

1. you don't need the if() statement checking for a totalcount of 0; the a!forEach() statement already handles an empty set gracefully.
2. Check whether your report is returning "people" (user or group) data or String data.  If it's returning String data then the code in the handler rule might not be interpreting it correctly.
3. to confirm, can you show the configuration you have set up for your "testgroupDisplay()" rule, including your rule input?  Screenshotting mine here again for reference.
   

screenshot of formatting rule.

Report is returning both user and groups.

I have tried multiple ways of getting group name ,everytime its giving me group id instead of name

Can you share the current definition for the Assigned To column of the report itself?  It seems like someone may have altered that - like if it's coming through as a single string, that will definitely throw off your ability to show it in your interface the way we're currently trying now.  It should look something like this:

FYI, i have a single App Market contribution to my name, and it's a SAIL task list with pre-built robust implementation of just this sort of thing.  So if all else fails, please feel free to download this and compare/contrast with what you're currently trying.
community.appian.com/.../advanced-user-task-report

Thanks Mike.. I just check the decimation for task assignee  and formatting - nothing was specified. I changed it to user or group name . But in my interface it’s still giving group ids

Other than that the definition is the same?  I.E. it's directly referencing the task_assignee_owner() rule with no other alteration?