It depends entirely on the group ride.

Some groups don't want anybody who can't keep up. Other groups would be happy to have you along even if you don't pull at the front. Other groups have designated leaders and they pull for the bulk of the time (I ride on one of those).

Whether you can keep the pace steady depends on the pace. If you can't, it's usually easier for the paceline if you get to the front, pull for long enough for the person who was in front of you drift back a bit, and then pull off and get back on the back of the line. That is less disruptive than pulling out at some other time.

Other groups may prefer you hang off the back, and have them circulate in front of you, though riding in the back is harder than riding near the front.

The best thing to do is to ask the group members when you should up.