First, get the date that they joined and convert to timestamp (strtotime function)
Next, get todays date in timestamp format (time function)
Then subtract current time from joined time
Now, number of seconds in one day is 24 * 60 * 60. Use this figure to calculate the number of days since the person joined.
Next, convert the number of days to a string detailing years, months, weeks and days. You can do this using division and modulo arithmetic (%) quite easily.