How to put validation on pickerfieldusers which can select max 5 users

Try including null check of the two variables

if(
  and(
    a!isNotNullOrEmpty(local!taskAdditionalAssignee),
    a!isNotNullOrEmpty(local!usersOfPickerB),
    contains(
      local!taskAdditionalAssignee,
      ri!record['recordType!{a6f5f29e-a89e-495a-afae-702df167aa19}.fields.{d0fec037-4f71-4d05-a7ef-766635d0ff13}']
    )
  ),
  "Assignee cannot be added as additional assignee",
  null
)

Thanks for your response but still not fulfilling my need as I have to store max 5 user but at backend level , field that should hold atleast 5 user is of text type.
is it possible my field is of text type and i have to store 5 user ?

I would advise you to create a new record called additionalAssignees, which will have one to many relationship with the existing record

field is already added in existing recordtype type and databse as well as additionalassignee wihich is text type but it h

but it has to hold max 5 user when selecting from pickerfieldusers

Use the variable to store the list of user selected in comma separated values.

Use joinarray() while storing and use split() while bring it back inside Appian.

It will be stored like userA,userB,userC,userD,userE

After splitting it will list of 5 text

Use the variable to store the list of user selected in comma separated values.

Use joinarray() while storing and use split() while bring it back inside Appian.

It will be stored like userA,userB,userC,userD,userE

After splitting it will list of 5 text

Can you please suggest how should I take new variable and store it as additional assignee as I have store in this way which is not correct 

Use a local variable to store all the selected users as a list. When you write to record, use joinarray(localVariable,",")

When you load the value from record use 

local!taskAdditionalAssignee: split(ri!record[recordField],",")

thankyou so much for your help
please help me how can I put validations as it is not woking by this 

validations: {
if(
and(
a!isNotNullOrEmpty(local!taskAdditionalAssignee),
a!isNotNullOrEmpty(local!taskAssignee),
contains(
touniformstring(local!taskAdditionalAssignee),
(ri!record['recordType!{a6f5f29e-a89e-495a-afae-702df167aa19}RECT Record Tasks Sync.fields.{d0fec037-4f71-4d05-a7ef-766635d0ff13}taskAssignedTo'])
)),
"Assignee cannot be added as additional assignee",
null
),

}

I have added touniformstring beacuse it was throwing error as-

local!taskAdditionalAssignee as user type and

(ri!record['recordType!{a6f5f29e-a89e-495a-afae-702df167aa19}RECT Record Tasks Sync.fields.{d0fec037-4f71-4d05-a7ef-766635d0ff13}taskAssignedTo']) as text

if you have any suggestion please let me know

error is something like this:

Interface Definition: Expression evaluation error at function 'contains' [line 275]: Invalid types, can only act on data of the same type (User, Text)

Try wrapping both variables that you are comparing with touniformstring()

not throwing any validation now-

validations: {
if(
and(
a!isNotNullOrEmpty(local!taskAdditionalAssignee),
a!isNotNullOrEmpty(local!taskAssignee),
contains(
touniformstring(local!taskAdditionalAssignee),
touniformstring(ri!record['recordType!{a6f5f29e-a89e-495a-afae-702df167aa19}RECT Record Tasks Sync.fields.{d0fec037-4f71-4d05-a7ef-766635d0ff13}taskAssignedTo'])
)),
"Assignee cannot be added as additional assignee",
null
),

}

is there any way we can convert text type to user type?

You can use touser()

still not throwing any validations-

validations: {
if(
and(
a!isNotNullOrEmpty(local!taskAdditionalAssignee),
a!isNotNullOrEmpty(local!taskAssignee),
contains(
local!taskAdditionalAssignee,
touser(ri!record['recordType!{a6f5f29e-a89e-495a-afae-702df167aa19}RECT Record Tasks Sync.fields.{d0fec037-4f71-4d05-a7ef-766635d0ff13}taskAssignedTo'])
)),
"Assignee cannot be added as additional assignee",
null
),

}

I tried wrapping up of both with touser