Hi, what does this does? Higher XP multiplier with more playtime, or less? Or people just get XP periodically?

It is in the front page:
XP is obtained by the TEAM that:
- Kill an alien by tower defense (based on the amount of its health and enemy evolution factor)
- By researched technologies (based on the cost of technology X enemy evolution factor)
- By rockets launched

By players that:
- Mine trees or rocks
- Kill aliens directly

That doesn't answer that option.

Sorry. I did not undertand at first. Now I got it: this mean if you Join a game that someone is already playing for 4 hours, you wont receive the same Xp as those players that are there since the begining. There is a % of xp show in the character panel to trace this. The more time u stay in that game, that % goes rising

There is no periodically xp.

Oh, I see. Thank you!