Q: RAID etiquette on a casual team

I’ve been on several casual teams in a public world and someone will occasionally enter a RAID to farm EN06, usually a team member, not the leader. I’ve also been doing this recently.

Is it possible to join a team member while he/she is in a RAID, or can I only join the team leader? Also asking for myself, in case someone suddenly shows up mid-RAID. I tend to stay away - don’t want to mess anything up.