<any_nick> !Add alpha
-Your_Bot- You are in the Alpha group
<any_nick> !groups
<Your_Bot> [Group] Alpha 1 users [Group] Beta 0 users
<any_nick> !Add beta
-Your_Bot- You are in the Beta group
<any_nick> !groups
<Your_Bot> [Group] Alpha 0 users [Group] Beta 1 users

Code:
on $*:text:/^!groups$/iS:#:msg # [Group] Alpha $numtok(%alpha,32) users [Group] Beta $numtok(%beta,32) users

on $*:text:/^!add\b/iS:#:{
  tokenize 32 $strip($1-)
  if $istok(alpha beta,$2,32) {
    set $+(%,$2) $addtok($($+(%,$2),2),$nick,32)
    var %2 = $iif($2 == alpha,beta,alpha)
    set $+(%,%2) $remtok($($+(%,%2),2),$nick,1,32)
    notice $nick You are in the $2 group
  } 
  else { msg # Group nonexistent. Use: !add alpha or !add beta }
}