From Archives of Nethys (Core Rulebook, pg 508):
If you choose not to keep the whole group at the same character level, you’ll need to select a party level to determine your XP budget for encounters. Choose the level you think best represents the party’s ability as a whole. Use the highest level if only one or two characters are behind, or an average if everyone is at a different level. If only one character is two or more levels ahead, use a party level suitable for the lower-level characters, and adjust the encounters as if there were one additional PC for every 2 levels the higher-level character has beyond the rest of the party.
Party members who are behind the party level gain double the XP other characters do until they reach the party’s level. When tracking individually, you’ll need to decide whether party members get XP for missed sessions.
My GM decided to penalize players who miss sessions by not rewarding them the XP they missed. This means unbalanced levels, but with double XP gain they catch up quickly.
Your GM should be giving the party easier encounters if your party's average level is lower and should not let you be more than a couple of levels behind.