Better late than never!

JavaScript (ES6), 98 97 bytes

Saved 1 byte thanks to Neil

Takes the list of guests l and the party time h in currying syntax (l)(h). Expects a trailing linebreak on the list. Returns a space-separated list of names such as Lisa Bob.

l=>h=>l.replace(/(.* )(.*)\n/g,(_,a,b)=>(t-=T(b))<0?a:'',t=(T=h=>eval(h.replace(/:/,'*60+')))(h))

Formatted and commented

l => h =>                         // given a list of guests l and a party time h
  l.replace(                      // for each guest in l:
    /(.* )(.*)\n/g,               //   extract the name a and arrival time b
    (_, a, b) =>                  //   subtract the arrival time from the time counter
      (t -= T(b)) < 0 ?           //   if the result is negative:
        a                         //     the guest is late: keep the name
      :                           //   else:
        '',                       //     the guest is on time: remove this entry
    t = (                         //   initialize the time counter t
      T = h =>                    //   define T():
        eval(                     //     a function that takes either a time
          h.replace(/:/, '*60+')  //     in hh:mm format or an amount of minutes
        )                         //     and returns an amount of minutes   
    )(h)                          //   call it with the party time
  )                               // end of replace()

Demo

let f =

l=>h=>l.replace(/(.* )(.*)\n/g,(_,a,b)=>(t-=T(b))<0?a:'',t=(T=h=>eval(h.replace(/:/,'*60+')))(h))

console.log(f(`Kevin 13:02
Ruby 5
Sam 3
Lisa 6
Bob 12
`)('13:15'))

PHP, 118 98 95 91 bytes

while($n=$argv[++$i])$i&1?$p=$n:($t=($f=strtotime)($n)?:$t+60*$n)<=$f(end($argv))?:print$p;

takes input from command line arguments (you may interprete that as lines separated by spaces if you like); prints names without a delimiter. Run with -r or test it online.

edit 1: saved 20 bytes with direct printing
edit 2: saved 3 bytes by removing the delimiter
edit 3: saved 4 bytes by exploiting that plain integers are no valid dates for strtotime

breakdown

while($n=$argv[++$i])       # loop through arguments, skip [0]
    $i&1                        # if index is odd   
    ?   $p=$n                   # then assign name to $p
    :   ($t=                    # else $t =
        ($f=strtotime)($n)          # if $n is a valid time, parse it
        ?:$t+60*$n                  # else add $n minutes to current $t
        )<=$f(end($argv))           # if $t <= parsed party start
        ?                           # then do nothing
        :print$p;                   # else print name

c, 178 bytes

main(c,o,d,e,g,O,l,f,x,y)char**o,d[80],*O,*l,*f;{for(sscanf(o[2],"%d:%d",&e,&g),x=e*60+g,l=";",f=o[1];O=strtok(f,l);f=0)(y=sscanf(O,"%s%d:%d",d,&e,&g)^2?e*60+g:y+e)>x?puts(d):0;}

Try it online