You may activate Guardian of New Benalia's last ability even if it's already tapped. You may also activate that ability multiple times in the same turn, even if Guardian of New Benalia already has indestructible.

You may tap only one creature for an enlist ability of an attacking creature, and a single creature can't be tapped for more than one enlist ability.

To enlist a creature, that creature must be untapped, it must not be attacking (even if it has vigilance), and it must have haste or have been under that attacking player's control since the beginning of their current turn.

When a player taps a creature for an attacking creature's enlist ability, that attacking creature gets +X/+0 until end of turn, where X is the tapped creature's power. This is a triggered ability that goes on the stack immediately after attackers have been declared in the declare attackers step.

Guardian of New Benalia's second ability triggers whenever its controller taps another creature for Guardian of New Benalia's enlist ability.

The attacking player chooses whether to tap a creature for an enlist ability immediately after they tap the creatures that they have chosen to attack with. You can't choose to enlist a creature later.