How do I group an SQL table by time stamps close to each other?

I know this doesn't give you a complete solution but it might help get you there, using row_number to identify rows to be grouped together in durations of 60 seconds:

with u as (
  select *,
    id - row_number() over (partition by userid, round(extract('epoch' from creationdate) / 60) * 60 order by creationdate) gp
  from t 
)
select 
  row_number() over(partition by max(userId) order by max(creationdate)) GroupNo,
  max(userid) UserId,
  min(creationdate) StartOfRange, max(CreationDate) EndOfRange,
  round(max(date_part('second',creationdate::time))- min(date_part('second',creationdate::time))) duration
from u
group by gp

window functions allow you to specyfy a range. You can use that to get the first value and last value for the addressed set of records. [I needed the omg subquery to actually use the values (substract them) and to filter only the (pseudo-) aggregates. Just omit the WHERE clause to see how it works...]

SELECT
        first_id, user_id
        , ze_first, ze_last
        , (ze_last-ze_first) AS timespan
        , 1+(last_id -first_id) AS nrecords
FROM    (
        SELECT
        id, user_id
        , first_value(id) OVER www AS first_id
        , last_value(id) OVER www AS last_id
        , first_value(creation_date) OVER www AS ze_first
        , last_value(creation_date) OVER www AS ze_last
        FROM user_session_activity_table
                WINDOW www AS (
                        PARTITION BY user_id
                        ORDER BY creation_date
                        -- This is the magic ...
                        RANGE BETWEEN '1 min' PRECEDING AND '1 min' FOLLOWING
                        )
        ) omg
WHERE id=first_id -- anything goes ...
        ;

You need STRING_AGG function in postgresql; some thing like,I am not familiar with postgresql

SELECT id,userId,STRING_agg(id,';'),to_char(creationDate, 'HH:MI') FROM public.user_session_activity_table

GROUP BY
    userId,to_char(creationDate, 'HH:MI')

This is my solution, it's VERY UGLY, but it gets the job done. As you can see I have used the LAG() function to deduce the start of a user session, and used another LAG() function in an outer query to deduce the start of a next session. Between these dates is the content of the group, which is joined afterwards. I hope this helps someone who stumbles upon this problem in the future. :)

As a side note, I'm still looking for better solutions, if anyone reads this.

SELECT 
    "usav"."userId" AS "userId",
    "groups_2"."groupStartDate" AS "sessionStartDate",
    MAX("usav"."creationDate") AS "sessionEndDate",
    MAX("usav"."creationDate") - "groups_2"."groupStartDate" AS "sessionLength",
    "u"."email" AS "userEmail"
FROM 
(
    -- group_2
    SELECT 
        "groups_1".*,
        ROW_NUMBER() OVER wind AS "groupId", 
        LAG ("groups_1"."groupStartDate", -1) OVER wind AS "nextGroupStartDate"
    FROM 
    (
        -- group_1
        SELECT * FROM 
        (
            -- sq
            SELECT
                "usav"."userId",
                "usav"."creationDate" AS "groupStartDate",
                LAG ("usav"."creationDate") OVER (
                    PARTITION BY "usav"."userId"
                    ORDER BY "usav"."creationDate") < "usav"."creationDate" - INTERVAL '5 min'
                IS DISTINCT FROM false AS "isGroupStart"
            FROM public."user_session_activity_view" AS "usav"
        ) AS "sq"

        WHERE "sq"."isGroupStart"

        ORDER BY
            "sq"."userId",
            "sq"."groupStartDate"
    ) AS "groups_1"
    
    WINDOW wind AS 
    (
        PARTITION BY "groups_1"."userId"
        ORDER BY "groups_1"."groupStartDate"
    )
) AS "groups_2"

LEFT JOIN public."user_session_activity_view" AS "usav"
ON "usav"."userId" = "groups_2"."userId"
    AND "usav"."creationDate" >= "groups_2"."groupStartDate"
    AND ("usav"."creationDate" < "groups_2"."nextGroupStartDate" 
         OR "groups_2"."nextGroupStartDate" IS NULL)
         
LEFT JOIN public."user" AS "u"
ON "u"."id" = "usav"."userId"
        
GROUP BY
    "usav"."userId",
    "groups_2"."groupStartDate",
    "u"."email"
    
ORDER BY
    "usav"."userId",
    "groups_2"."groupStartDate"